Diesel spec 15w40 is ideal for flat tappet cam high performance engines especially those with solid lifter flat cam applications. Use diesel spec HD30 in your old GM, Ford and Chrysler engines, those before catalytic converters. The best engine assembly lube in my opinion is STP oil treatment. It's cheep and it is loaded with ZDDP. Great for initial start up and cam brake-in.
No .. no .. sorry: Race engine tolerances are virtually ALWAYS much LOOSER than any factory street engine meant to last hundreds of thousands of miles. [Recall 'Smokey Yunick' and his 'barely together' motors which were magically beginning to come apart at the finish line after winning the race.]
Sorry, Mr. Smith, but I beg to differ. The GM-built crate flat tappet crate engine in one of my lightly-used classic trucks ate three cam lobes and lifters after 24,000 miles and fifteen years of use. The oil was changed twice a year regardless of mileage, and the thing still bit me. I replaced the cam and lifters (upgrading to Crower CamSaver lifters in the process) and added Valvoline Premium Blue oil. The amount of ZDDP in non-diesel spec oils is most definitely inadequate for old flat tappet gasoline engines. And the ZDDP additives must be used with caution, because too much can coat the cross-hatch on the cylinder surface, causing excessive oil consumption. Racing oil with a high (relatively speaking) level of zinc would be better than a non-racing lubricant when looking to keep a 60's or 70's era engine alive.