I recently changed out both front and rear axle lubricants, and installed Mobil One 75w90 GL5 synthetic, sealing the covers well with Permatex Black. For anyone who hasn't changed out the OEM petroleum product, be prepared for a thick, nasty, black mess when you do it. The break-in process really contaminates the lube, and I only wish I'd changed it out earlier than 15k miles. 75w140 didn't seem warranted, as I only tow occasionally, and the stuff is very expensive at $30 a qt. compared to $10/qt. for 75x90.
I didn't install the recommended 'friction modifier' as I have heard that it is not needed when using synthetic lubricant. I did a few figure-8's afterward, and I can't hear any chatter at all, in fact, it drives and sounds perfectly normal.
I've also heard that Mobil One has modifiers already added in their LS line. The bottles I bought were the old-style 75w90 style without the LS identifier. Perhaps this is a newer product in older packaging?
The question I have is this: does this friction modifier make the lubricant more, or less slippery? My guess would be less, so that locker clutches grab better, but if this were the case, wouldn't the differential make more noise if extra limited slip additive were installed?
Second question: does the addition of LS additive cause differential components to wear more quickly?
Third Question: assuming that no limited slip additive is present in this product AND the differential clutches are not chattering AND the limited-slip feature is working properly (I plan to check function the next time I lift the rear-end), is it safe to continue using this until my next lubricant change?
Incidentally, I changed out the transmission and transfer case fluid as well; both came out squeaky clean. The TC got ATF+4, the 6-sp. transmission got Royal Purple synthetic MTF.
As I understand it, synthetic 10w30 motor oil could also have been used, but I haven't heard of anyone actually using it. Safe to use, not recommended, what? (I get this information from wikipedia.com, as there is very limited information on recommended MTF types out there; some even suggest that the $35-bottle OEM fluid is nothing more than 10w30 motor oil.)