Mobil One, Anyone?
 

With an auto conversion powered airplane, is there any reason not to use a good synthetic oil? Are synthetics compatible with 100 LL? If not, why not?

Don't
 

What about Aeroshell's synthetic blend 15-50 oil?

Mobil One and "alternative" engines
 

From all I have read over the years the synthetics didn't work reall well in aircraft engines due to the greater amount of blow by associated with the looser clearance of rings on an aircooled engine, etc. Many of my rotary buddies (Mazda 13/20B engines) run Mobile one and 100LL or 87 octane car fuel without any issues and that is what I will use in my 13B when I get it flying...

Doug Lomheim
RV-9A; 13B / FWF (on hold for now)
RV-3 O-290 under restoration (former GPU...sorta of an "alernative engine" :)

Anyone that has been around aviation a while would not use a 100% synthetic oil in an engine that uses 100LL if you gave them the oil free. Too many engines were destroyed by Mobil 1 aviation oil when it was on the market. The synthetic oil does not handle the lead well and the sludge that formed destroyed the engines.

Do NOT use synthetics
 

Bud Warren told me to never never use synthetics with 100LL. The lead will turn it to a gray sludge and destroy your engine. Use SAE 30W or 5w-30 and keep it clean. Period.

Leaded fuel + detergent additive package = Bad
 

Myron,
More important than the issue of Mobil 1 being synthetic is the fact that it is a DETERGENT automotive engine oil. [FYI, almost all automotive oils are detergent oils] Automotive detergent additive packages are NOT compatible with the use of leaded fuels.
Aircraft fuels are "ash-less dispersant". This means that they are designed to let the lead settle out of the oil and sink to the bottom of the sump. Use of Mobil 1 would be a great idea for your auto engine IF and ONLY IF you stick to unleaded fuels, which is what automotive engine oils are designed to use.
FYI, for those mentioning that Aviation Mobil 1 has been discontinued, the issues of that product would not be relevant here. The problem with the Aviation Mobil 1, was that it leached copper out of the engine. This seemed to be an issue with Continental Engines, more so than with Lycoming products. Since Myron is using an auto engine, not relevant here.
Charlie

So the primary fuel in use will be 93 octane mogas. And the engine most likely will be a Chevy. Considering that on occasion we might be limited to 100LL, what oil would be best suited for this application. And what about Aeroshell 15-50 synthetic blend?

Well let's just say the word never got to all of us
 

I use Aeroshell 15W-50 exclusively because of a consumer report years ago that said it was the only aviation oil with an anti-wear aditive that was verified to work by testing. When I changed to a Blended airfoil prop a few years ago I was surprised by the thick gray pudding consistency material covering everything in the end of my crankshaft. I assumed this was normal. I cleaned it out with a rag, installed the new prop and continued on.

The engine runs fine. I run it WOT all the time in cruise flight never less than 2450 rpm. I have a little over 800 hours on the engine and yesterday clocked a little over 215 mph in the MERFI 150.

My question to Gary Sobek, who I trust completely, is "Is this an example of what you were talking about and is there a demonstrated failure mode from this characteristic?"

Bob Axsom

Myron,
This would be a much better choice, if 100 LL will be used. It has the correct "ash-less dispersant" additive package.
Charlie