TCP. Tricresyl phosphate. It's a lead scavenger that has good anti-scuff properties. See
http://www.avblend.com/faa/kas_thomas_lenckite.html
I used to use 15W40 auto oil in my A-65 until I read somewhere that auto
engine oils typically have stronger detergents and scrubbers in them that can loosen the accumulated crud in the case and send it all to the filter and overwhelm it. I did find lots of carbon and varnish in the screen after that and went back to aircraft oils. I think if the engine was fresh and not loaded up with crud the auto engine oil might be OK.
I find that the Aeroshell 15W50 runs far cooler than the W80 I used to use. It reduces friction so much that getting the oil temp up now is a problem. I checked the gauge and it's OK. I'm going to have to put a blanket on the tank--even in the summertime--to get the temp up to 180 or so to keep the moisture out.
Dan