<<What's the difference between aviation and automotive oils, other than
price?  Any reason we shouldn't use automotive oils in experimental
aircraft?

Naf
LNC2 N7PN>>

I'll take a stab at this one, although I'm not a chemist.  I was told that
the main difference is that the aircraft oils are "ashless", meaning that
when they burn they leave no ash left over.  If an engine burns a
significant amount of oil the ash could collect on the plugs and cause
fouling.  At Mercury Marine we were trying to use the initial Chevy 502's
and found we couldn't keep them running for very long at high power because
of this - the initial bore/ring combination was very poor and they burned a
lot of oil.  We switched to aircraft oil and the problem went away.  The
word "dispersant" is another word for "detergent" so the normal "ashless
dispersant" oil is both ashless in composition and has a detergent additive.
Presumably if your engine burns little or no oil you could get by with using
automotive oil, but most aircraft engines burn significant oil.  How would
you know it is okay?  That's the rub as we found that the plug contamination
was conductive at high temperatures, so the engine would quit running one
cylinder at a time.  Not just a "rough run-up" symptom that could be
corrected on the ground.  So you have a bit of a conundrum - if your engine
burns a lot of oil you need the high-priced aircraft oil.  If your engine
doesn't burn much oil the high price doesn't mean very much, but then you
can use the cheap oil.  Might as well stick with the aircraft oil in either
case.

Gary Casey
ES