The limited slip I have in an Auburn HP Series, which uses a cone clutch. There is no harm in using a synthetic with that unit, assuming it has sufficient FM characteristic either naturally or added, it will just deliver a reduction in the bias ratio. How much, I do not know. I may soon find out.
I had an inside source at Eaton who once told me that some syns cause problems with Truetracs and some do not. The problem is, they really have not done any studies to determine which is which... at least they claim not to have... and if they know exactly what is causing the problem, they would not share it with me. It's simply easier and cheaper for them to continue to recommend "regular" mineral oil. If a customer ignores the lubricant recommendation, whatever happens is on them. One of the most common questions on the Eaton Tech line concerns sythetic lubricants.
All that said, there are many people running syn in Truetrac diffs. I ran Amsoil Series 2000 in mine with no particular problems and now run the equally slippery LE607 mineral oil (bolstered with a lot of a Moly-like additives) in it, also with no adverse symptoms. WIth some syns, including the factory fill oil Ford puts in the late model F-Series trucks with the S-110 axle that has a Truetrac. The adverse symptom is noise, a squealing in particular. There may be a reduction in bias. Don't let the oil issue stop you from getting a Truetrac, Colton. Even if you have to stoop to a mineral oil. Do Like I did and experiment a little once the unit is broken in on mineral oil.
My opinion as to why Eaton recommends mineral for the Posi is the same. That recommended lube is the one they used for testing to obtain the advertised bias ratio. SInce oils can effect performance and driveability, again, they would rather not experiment with every type of oil to see what works and what doesn't. Much easier to recommend what they know works
