Originally Posted By: RED_V6
the gear oil is for manual transmissions....
That's not what it reads; it cleary is referring to the transfer case, on both page 9-3 and 9-8, in reference to ATF and gear oil. Now - it does also state to use gear oil in the manual transmission, but that is not the topic we are discussing. You started this thread about the t-case. And that's what I looked up in that owner's manual link you provided. Those two pages clearly state the use of GL-4 75w-90 is acceptable in the xfer case. But the D-Matic (or DEX/Merc alternative) fluid is also clearly spec'd.
Regardless ...
If it's a chain driven transfer case, they typically don't really care which fluid it used.
There's a little-known debate in the GMT-800 world where some folks prefer 5w30 motor oil over ATF, only because one guy suggested it in a trade magazine. After personally interviewing him, I wrote up a short article about his suggestion, and how the rest of the market did not see it his way. My point? There are folks that use motor oil in lieu of ATF as well, and typically nothing bad happens.
Ironically, in the Harley Davidson world, where the primary case houses a chain-driven system, they spec what is essentially a motor oil, and yet some of the people to believe that ATF is "better", so they eschew the recommended fluid in favor of an unapproved alternative.
My point? The grass is always greener on the other side of the fence ...
Generally, chain driven xfer cases just are not that sensitive to fluids. As long as they are reasonably close to the spec'd vis, an ATF, GL-4,5 or engine oil will work. There are hundreds of thousands of examples out there running around right now with some variation of these options, and the t-cases are not dropping out of the frames because someone used brand "X" of product "Y" instead of option "Z".
One of the benefits to using a syn is that they may resist evaporation a bit "better" than a conventional lube, and therefore if you're inclined to ignore the fluid (as some folks are) then they longevity of syns may pay off. The 261/263 t-cases in the GMT-800 trucks tends to run hot at times, and so the syn can be an advantage. I use syn ATF in this application for this reason. However, if you're diligent about doing proper PM and checkinf fluid levels, this is a moot point as you'll (hopefully) never let the fluid get low enough to warrant concern anyway.
Do as you see fit. You have all the info you need (starting with your owner's manual; the most important source) to make a decision that "best" fits your situation.
