I received some excellent feedback from a Corvair expert via email too:
Often after overhaul, mechanics fail to re-install a metal air deflector under each Corvair cylinder head bank. This piece of metal is held into place by two small wire clips, and is hard to re-install. Clips are a pain, and clip onto the metal rods that hold on the cylinders. This deflector is required to direct air flow onto the heads. If these two pieces of metal are missing, heads will over-heat.
The pieces of metal can be seen through the open bottom air exit doors when you open them. Metal pieces (2, one on each side) are about 18 inches long and about 3 inches wide, and are "waffled" to fit under the cylinders. Corvair owners must use 30 weight conventional oil. Never synthetics or mult-weights. 30 weight motorcycle oil is good to use.
I am an old Corvair expert. I have raced and owned over 30 Corvairs in my past. I had over 11 at once.