cancel
Showing results for 
Search instead for 
Did you mean: 
Hagerty
Hagerty Employee

What's the real difference between synthetic oil and conventional oil? | Hagerty Media

Nothing ignites the vintage car discussion boards like the oil debate, and the hardest part of that conversation involves separating fact from fiction. Many long-held beliefs may not be based around modern science, but sometimes the knowledge to correct those false assumptions is hard to find.
https://www.hagerty.com/media/maintenance-and-tech/whats-the-real-difference-between-synthetic-oil-a...
66 REPLIES 66
JeepCJ5
Intermediate Driver

Just Monday I was on the hunt for some gear lube that would work in my recently acquired 1986 F150 4x4 with a NP435 manual trans. Internet research told me that GL-5 gear oil isn't yellow-metal safe, and it seems plausible since after a couple of years of running GL-5 spec gear lube in my CJ-5 the syncros got a whole lot worse. But browsing through the shelf at several of the local auto parts stores didn't turn up any GL-4. Asking the folks behind the counter resulted in the typical no-help response. Finally I stopped at a NAPA and the guy there quickly located some in the catalog and had it for me the next day. I'm not sure if I've gotten all worked up over GL-4 vs GL-5 for no reason, but I hope this Sta-Lube 85w-90 treats me right with this truck.
mikem350
Pit Crew

You can still find synchro safe oil. Search for manual transmission Gear Oil. I used Pennzoil.

Most say meet GL-4 spec.
Ajakeski
Intermediate Driver
Studenorton
Advanced Driver

I called AmsOil's tech line once or twice with abstruse questions so far out that if I'd gotten them I'd have sworn they were "stump the band" material. I got answers from their research staff, not just a crib sheet -- and they sent me somewhere else to spend money. So I recommend their advice as trustworthy.
Evan67
Pit Crew

I use PennGrade GL-4 gear oil in my toploader. It's available at Summit and other places.
Guitar74
Gearhead

All of that info, but a picture of a brand that has been stigmatized for years as having the crappiest oil. I would even use Quaker State to coat a wok, let alone trust it to protect my engine.

 

JeepCJ5
Intermediate Driver

Just out of curiosity, what issues have you had personally with Quaker State oil?

50s60s70s
Intermediate Driver

I've not had any experience with quaker state oil..but after rebuilding several Penzoil engines vs engines that ran Valvoline And Mobile products the Penzoil engines had significantly more wear and sludge internally.
The article didn't mention the addition of ZDDP(zinc phosphate) and why it was used in flat tappet engines and the reason for removal in modern oils.
MoparMan
Advanced Driver

With respect to Quaker State Oil: I own a '74 Challenger w/ 69k actual miles on it. When I first bought it, I changed the oil every 3k religiously , using QS oil. Imagine my dismay, when upon removing a valve cover finding that the internals were coated with a black sludge! I stopped using QS and switched to Pennzoil or Castrol GTX. As far as synthetic vs. dino, I use full synthetic in my newest car, and semi-synthetic in the older ones, modestly extending the change intervals. YMMV! 🙂
Numberscruncher
Detailer

Now there's a useful article. Thanks.
mwmyers91
Detailer

Great article really, good basic info, brand preference is probably more marketing than real difference truth be known. But thats why they make 32 flavors of ice cream 😄
cestor01
Intermediate Driver

Yeah, yeah. All of that is interesting. The real importance, however, gets lost in all the debate about synthetic vs natural oil. That is - What do you need to protect your car? If you plan to not change your oil for 20,000 miles, use synthetic. If you are going to change your oil when it gets dirty, use the cheapest oil (or anything else you want) because any oil will have good viscosity for that long. I have put nearly 500,000 miles on several cars, my oldest current car is a 1964, and I have never had to replace an engine, or rebuild and engine, or junk a car, etc., for anything even remotely related to something that could have been prevented by using better oil.
Studenorton
Advanced Driver

I have several different opinions, but I've heard a couple of extremely experienced mechanics say that modern ethanol content creates so much ash that all other oil longevity is compromised, and only frequent changes make up for the change. This agrees strongly with your argument.

Now, for those who never use any fuel with ethanol in it, other factors may come in.
MoparMarq
Detailer

Been using Amsoil in everything from MY '72 to MY '09 with no problems. All 5 cars over 100K miles each, 2 at over 200K miles. Main reason for the switch (back in the '90s) was a for a small improvement in fuel economy, but now the reason (besides the longevity and fuel economy) is for reduced oil change intervals. I change the oil in all of them every September. Doing oil changes now for me is like doing yardwork; I can't stand doing it, but it's gotta get done.
50s60s70s
Intermediate Driver

Kyle, please correct me if I'm wrong but if I remember my chemistry, synthetic oils break down once their critical temperature is reached loosing their ability to maintain their viscosity ratings even after cooling. A conventional oil can reach its boiling point and will retain some protection to engine parts as it will not degrade significantly once it cools.
A synthetic last longer and provides more protection under normal conditions.
Tim
Instructor

In true internet fashion, only 5 comments separate complete contradiction of ideas. One person says any oil is fine, another says this oil sucks. One thing is true: it's always possible to find a supporting opinion in an Internet oil discussion! 😁

tahend
Detailer

I say just run your car with no oil then you don't have to rack your brain trying to make the right decision.
TimeFixers
Pit Crew

Do not use synthetic oil to break in your new motor.
10izboy
New Driver

One could discuss the relative merits of synthetic versus conventional oils ad nauseam. As someone who worked in the lubricant industry for many years, it is my view that the only reason for running synthetic oil is to get better low temperature performance. Specifically if you live in a cold climate and leave your car outside, your car is more likely to start in the morning. One can make the argument that this leads to lower engine wear, which is factually correct but in all likelihood has minimal real world impact. All reputable oils pass a series of ever more stringent tests to demonstrate their detergency performance, wear performance and anti-oxidant performance. If you change your oil regular the particular brand you use is of little importance.
Over time the base oil used in synthetic vehicles has changed significantly. At one time PAO's (Poly-alpha-olefin) were used exclusively; however in the last couple of decades the industry accepted that highly hydro-treated conventional base stocks could be declared "synthetic" due to the more uniform structure of the molecules and these have become prevalent as the base stock of choice in synthetic oils. The difference in cost between PAO's and hydro-treated base stocks is quite significant; however the difference in cost between more severely hydro-treated base stocks and conventional base stocks is relatively small. I could go on, but I am sure you get the point.
DanC
Pit Crew

In the early 80's, I sold Mercedes Benz. They Bragged about how long the cars lasted but HARPED on changing oil 3000 mi for diesels and 5000 for gas engines. The regular routine maintenance was the trick. We based trade in values on whether the maintenance book was filled in on time. Conventional or synthetic, I have 4 cars that get oil changed once a year or every 3000 miles, whichever is first. CHEAPEST insurance I ever bought. Highest mileage car is a 5.7 in a 99 Tahoe with 270,000 miles and compression is within 5% of new spec.
Buy whatever oil you want...but CHANGE it religously.
gary21sn
New Driver

The article reads like the writer is pro-synthetic and anti-conventional oil. How about posting an article on the subject that is unbiased and unopinionated. Hard to find those these days.
Guitar74
Gearhead

Or maybe talk about the lack of zinc in modern off the shelf oils that will help to give you flat lobes on a flat tappet cam. I have used both, and settled on Shell Rotella in my classics. High zinc content, lower price, and is more than good enough for a gasoline engine when you consider that it is a go-to for MUCH harder working industrial engines.

Solosolo
Intermediate Driver

Isn't Shell Rotella oil made specifically for diesel engines?
Guitar74
Gearhead

The reason SO many flat tappet guys, including myself, use it is because you no longer have to buy an additive on TOP of oil to get the pressure additives necessary for a flat tappet engine to work without wiping the lobes. This is ESPECIALLY critical when you are running higher valve spring pressures which are pretty much necessary with a decent cam (not necessarily radical, but with the more aggressive opening and closing ramps). So to answer your question, yes, it was designed for industrial engines. But think about the following: Industrial engines see MUCH higher stress/loads placed on them. Now think about how little of a load your typical classic car engine is put under by way of comparison. 

 

More succinctly put, your bearings, cam lobes, oil pump, have no real idea that diesel engine oil is going through it with the exception of having the zinc levels back to what they were and SHOULD be if you are running a flat tappet cam. Roller cam engines don't need as much ZDDP as flat tappets because a) the cam is much harder and b) you now have a roller following the cam lobe. The same rule applies for overhead cams engines as well. No need for the higher ZDDP levels as your cam follower is usually a roller design.

 

Also, the reduction of ZDDP was due to the fact that it is much harder on catalytic convertor. 

stevecobb45
Detailer

I've used Mobil-1 synthetic oil in virtually everything since 1984. If you don't believe the science then that's fine. I've also run several vehicles over 200K miles w/o any lubrication problems. My 1985 Arien HT16 garden tractor is still running too. I don't change my oil as often as I should but when I do I always use a new quality filter. I don't use it for break-in of a new engine because it lubricates so well, it prevents the rings from seating in. Factory engines don't need breaking in so I wait & change to synthetic at 500 miles or so. Cold weather start-up is where most all wear comes from in an engine & synthetics are just superior at all temperatures but especially cold start-ups.
Callen
Pit Crew


When you drive the supposed 15K miles a year synthetic is great, but being retired, and during covid, I've put about 350 miles on my F150 and still need to change the oil at 1 year intervals to maintain warranty. It get's expensive. Since most folks on the site a collector car people, a discussion of synthetic use in older cars would be a good addition to your article. Of course, you when you get into older cars the flat tappet and ZDDP discussion begins and will digress into discussions on which conventional oils are usable. It's a slippery slope slope. For now the TR6 and MG get Valvoline 20-50 racing. In other interesting topic on oils, why does Ford use a conventional/synthetic blend oil? Is it just cost, or is there a oil performance aspect? Seems like the conventional oil would degrade the performance of the synthetic.
OldCarMan
Instructor

No discussion of "Semi-Synthetic" oil was mention. Seems a bit under explained overall, too. Good start, but let's be comprehensive!
kimb
Pit Crew

Put me in the Rotella camp- suppose any diesel rated oil will work- get the xtra zinc for the classic. car- then if you want to go down the worm hole - what adjustments do you make if the tranny and engine share the same oil? T4 or T6 That is the question...... '63 mini
bechap
Pit Crew

Conflicting reports on my '53 Olds Rocket engine. One says conventional Shell T4 and the other Synthetic "High Mileage". Unfortunately this article didn't mention the difference in the "high Mileage" synthetic that makes it better for older engines. I suspect that it has something tom do with the larger clearances in the older engines. It's also supposed to help with the leaks. I'm going with the Synthetic high mileage till I hear something that changes my mind.
rcmsr
New Driver

i was lead to believe that with the synthetic being more slick so to speak and the old gasket material being what it was in the day and the rubber was a different compound,the synthetic would make more oil leaks. i try to stay with the conventional oil unless the rubber seals, such as the front and rear seals, have been changed to our new rubber material. From what i understand there is more zinc in the rotella than in the other oils. i am open to learn more about this. thanks
ScottC
Intermediate Driver

From what I've been told by a mechanic friend of mine who is also a boat nut that newer oils are designed to support catalytic convertors and they have a low zinc rate which is hard on bearings because that zinc actually aids the bearing slickness. He uses Rotella with zinc in everything he owns without a convertor and has no problems with it. Just what I've been told, I'm no engineer or anything but I do work in the oil industry.
mrfriendly
Pit Crew

Zinc (or ZDP, ZDDP) or Zinc dialkyldithiophosphates is an antiwear agent. It has been used in engine oils for decades. Almost all engine oils use this antiwear agent because it is less expensive than other antiwear agents. It is the phosphorus component in ZDDP that impacts the life of a catalytic converter. The industry has been reducing the amount of phosphorus in engine oils for nearly 30 years to increase life and reduce size of catalytic converters. Also, with flat tappet cam shafts being relegated to "as built" engines for collector cars for the most part, ZDDP levels for modern production engines is fine. Too much ZDDP can be chemically corrosive to engine parts. There is an SAE article which addresses this issue. Be careful with engine oil additives. They are generally not recommended by the OEMs.
Zephyr
Advanced Driver

Does anyone else remember paraffin-based oil, or recycled oil? I used both in my Corvair of long ago that was so worn out that it used a quart or two every 100 miles. I literally bought it by the gallon, at Grand Auto.
GRP_Photo
Instructor

I had a '61 E-100 with a bad rear seal. I bought multiple cases of recycled oil at a time for that thing. I finally swapped it for a Honda 50.
Bmike
Detailer

I've only used Castrol 20-50w dinosaur juice since my MG Midget days back in the 70's. Put it in everything (even the lawn mower), and change it according to manufacturer's instructions. Works fine, lasts a long time.
ScottM
Pit Crew

I’ve used Mobil one in my corvettes as suggested with great results in all weather due to the inherent characteristics. I found that in vintage high mileage engines it can leak and be consumed as tolerances are lower. The fact it flows better and resists sludge formation has made synthetic a perfect solution for all our cars from Chryslers to Toyota’s which are sludge prone.
mrfriendly
Pit Crew

Sorry, I can't help myself.
1. The word "Synthetic" is a marketing term and has no technical meaning with regard to engine oil (see Mobil v. Castrol, NAB circa 1998).

2. There are 5 groupings for base oils; Group I - >0.03 Sulfur (wt.%), <90% Saturates, and a VI between 80-119; Group II - less than or equal to 0.03 Sulfur (wt.%), greater than or equal to 90% Saturates, and a VI between 80 - 119; Group III - less than or equal to 0.03 Sulfur (wt. %), greater than or equal to 90% Saturates, and a VI greater than 119; Group IV are PAO's; Group V is anything not in the previous groups (e.g. Naphthenics)

3. For modern engine oils the base oils used are Group II, III, or IV. They can be used in combination or alone to make an engine oil.

4. Marketers can identify their engine oils almost any way they wish (e.g. Conventional, Synthetic, Semi-Synthetic, Synthetic Blend) to satisfy a certain market segment. It really tells you NOTHING about the performance capability of the engine oil.

5. What does tell you about performance capability is the specifications they claim to meet or are certified/licensed/approved to meet. Claiming a certain performance standard puts the burden on the consumer to believe the claim being made. Certified/licensed, on the other hand, means that an organization or OEM has accepted that the oil meets their specification. In my opinion, certified is much stronger than a claim. However, certified/licensed is only as good as the specification testing and enforcement by the body (Organization or OEM).

6. API licenses oils to "S" (spark ignited) and "C" (compression ignited) engines (e.g. SP for gasoline engines and CK-4 for diesel engines). OEM's can either recommend their own specification (e.g. Mercedes-Benz, General Motors - dexos) or API or ACEA for European Manufacturer's.

So, in summary, make sure you are using the correct viscosity for your engine and second choose the oil that is certified to the specification listed in your Owner's Manual.
gerry1936
New Driver

I've used synthetic oil for years. I change the filter every 3000 miles and top off with new synthetic. Do a complete oil change every year.
Lee
Pit Crew

I've owned a '07 F150 w/5.4L engine since new and always had the Valvoline Quick Lubes use the Valvoline™ Durablend™ Synthetic Blend oil. So when I got it paid off (89k miles), I decided to splurge and use the Full Synthetic for my next oil change. What a disaster! Whenever I backed up or moved at slow speed the engine sounded like a diesel on steroids! I read online about $2200 repairs to the Variable Cam Timing mechanisms or whole engine replacements.

I was able to put two and two together and figure out it had something to do with the VCT control valves in the heads. After I bought replacement valves, I notice the very fine screens they had. I the mean time, I called the local Ford service department and explained my predicament. To my amazement, he told me to put a quart of Marvel Mystery Oil in it.

So I switched back to the Valvoline Blended oil and added a quart of Marvel Mystery Oil. To my amazement, in a couple of days my truck was back to normal. I can only guess that the full synthetic oil somehow loosened up accumulated sludge and it plugged up the screens on the VCT valves. I hope you were not one of those that paid thousands to have your F150 fixed, when a quart of Marvel Mystery Oil could have done the trick!
TikiBarJim
New Driver

I started using Mobil 1 after an article I read by Smokey Yunick the famous engine builder & he said he built an engine & raced for a season when he tore it down to inspect it he said that there was hardly ANY wear at all on the internals. That’s when I started to use Mobile 1. If it’s good enough for Smokey it’s good enough for me.
As for QS I would never use it as I have seen too many engines valve covers that sludge up with that oil requiring a complete overhaul.
Steve_F
New Driver

When I was a kid (13 or 14) I had a Honda ATC185 I was trying to sell but it smoked like crazy. An old-time mechanic gave me some advice to use Vegetable oil in the motor, he said the oil would still burn but the smoke wouldn't be visible. I tried it, it worked, but everytime I fired it up it smelled like French Fries!!! I did get it sold that way, and a few weeks later saw it out in the local desert providing amazing fun, but smoking like crazy again!
Tinkerah
Technician

Genius!
okfoz
Intermediate Driver

The best description of Conventional vs Synthetic Oils I have heard is a Synthetic oil is perfect, all of the molecules are perfect bearings like you show above. However conventional oil looks more like Nerds Candy, a bit more organic, not quite round but close to round, and all a little different size.
Ajakeski
Intermediate Driver

I enjoy reading people's forum posts about how they disregard the factory engine oil recommendation and substitute what they think is best.
Punk
Detailer

I 'got into it' with a mechanic friend recently. I was sticking with the thermostat/cooling fan settings recommended by the factory. He claimed that modern oils work better at a higher temp. My response was to say that Jaguar knew what they were doing when they wrote the manual and recommended oils and running temps. He said I was relying on 50 year old info and things have changed. In the end, its my car, so we are running what I want. But I wonder. Is it like putting in brighter headlights or an alternator instead of a generator to 'update' oil recommendations?
Pete
Pit Crew

I'm surprised there's no discussion of "Dexos approved" synthetic oil (required in GM cars and maybe others) vs. other synthetic oils.
mrfriendly
Pit Crew

What would you like to know about dexos?
jrygg
Pit Crew

Much of the conversation here is about fairly modern engines. I have a 1960 Sprite and run 20-50 conventional oil (only drive in the summer) I add Rislone ZDDP supplement, and only use non ethanol fuel to which I add Motor Medic Lead Substitute. So far this has worked fine for me. I also use almost the same technique in my 1966 Ford Bronco, other than using 10-40 conventional oil. I will never use ethanol fuel in any of my older engines. In my modern cars, I use GM's synthetic oil with good results. My inboard boat has a warning label advising against the use of ethanol fuel, but who knows how long Big Brother will allow the use of non ethanol fuel?
TG
Instructor

I knew molecule uniformity was part of the oil but i never knew that distilled, fractioned, or whatever they do with it conventional oil still had so much randomness in molecules
I will say that one of the old folks tales about synthetics is they tend to promote leakage in older engines, which I can attest to. I put synthetic in my older Cadillac motor because that is actually what it is supposed to take, and it's not leaking bad enough to leave stains, but I can definitely smell it. I may revert back to conventional on the next oil change
My personal experience has taught me that keeping the crankcase full and changing oil frequently are much more important than oil type provided you aren't going ridiculously outside of manufacturer recommendations