My original post did not suggest I was going to use motor oil, but merely trying to find out why a modern highly developed motor oil would be inferior to aero oil in an engine more akin to a tractor engine (and they work all day at high power settings). In UK we can use unleaded motor petrol in the small Continentals, subject to fuel pipes being able to cope with the additives, and I find it difficult to see a problem with the steel, alloy and gasket materials in a small aero engine handling the chemicals in motor oil. The "just don't" brigade do not provide any answers. As far as slick 50 is concerned, a local truck fleet collegue started using it about 10 years ago, and the almost immediate benefit was an increase in cold starting rpm by the starter motor, which does suggest a reduction in internal friction. I put it in my Rover 75 CDT (after the warranty expired) and had an almost 5mpg reduction in fuel burn. It is also in a BMC engine in my boat, with seeming improvement in starting.
My aim in this thread is not to argue, but just to get an interesting and maybe informed discussion.