Diff oil and transfer case fluid for 1999 Suburban

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This is my first post and I'm so glad I can be apart of BITOG.
My friend just bought a 1999 Chevy Suburban 4x4 1500.
He's not much of a preventive maintenance person when it comes to his 6 vehicles, 4 of which are Suburbans (79,85,89,99). He doesn't even wash any of his trucks. I hope I can get him to wash his 1999 because all that dirt just causes rust around the wheel wells.

On to my questions. I think both differentials need their lube changed but not sure what to use. What gear lube should I use? I'm thinking Mobile 1 75-90.

I don't know what fluid the transfer case takes or how much to use. But I do know it's electronically operated.

Any Advice on how to change either of these would be good but don't really worry about it.
 
Get the manual that is stashed away in the glove box. Look up the fluid section. If it calls for SAE90, SAE85w90, SAE80w90, or SAE75w90, Mobil1 75w90 will be perfect. If it calls for SAE85w140, SAE 80w140, or SAE 75w140, buy Mobil1 75w140.

Transfer cases wary widely in the fluid type they take. Check that owner's manual, and if it says to use Dexron I, II, or III, fill it with Dexron III or VI. If it says gear oil, use the correct viscosity. Sometimes the transfer case takes a dealer-only fluid. In those cases, don't take chances, buy the dealer-only fluid.
 
Many of the electronically shifted transfer cases on the GM 4wd trucks take a fluid that is available at the dealer, and it aint cheap.
 
Thanks molakule, I'll tell my friend that.
I found the name of the fluid you guys are talking about. It's called Auto-trak II and people said it's about $6-$9 dollars a quart. The case takes less than two quarts. I think it was used in place of ATF in gm transfer cases with an auto 4x4 mode. Using the auto-trak fluid in these transfer cases eliminates unwanted clutch grad or whatever you call it. According to wikipedia it was an added option on the suburban starting in 1998.
I'm not quite sure how much of what I said is totally accurate.
 
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