A 1970 was designed for leaded fuel, was it not? The Avgas makes it run considerably smoother than premium unleaded. Actually, AVGAS 100LL is a reduced lead variant, and the octane rating is really 110 octane. A little bit every now and then couldn't hurt, right?

I don't know if I'm imagining things or not, running an AVGAS mix seems to make the engine run cooler, too.

I read the owner's manual for my car, and the recommend minimum octane rating was 96, and about the only place to get that is at a general avaition airport. In fact, I have run both 92 octane and 94 octane fuels in the car, and it certainly runs smoother at 94 octane, but it runs best with a 20 % AVGAS mix. I certainly would not run 100% AVGAS becasue I wouldn't be able to get the spark plugs out without using an anti-seize compound like on an airplane engine.

Also, you have the 280SE with the V-8, don't you? I have the straight-six. That might make a difference in which octane rating to use?