Changing fluids

I want to go ahead and change the fluids in Rear and front differential and the transfer case, what kind of oil to use ? For the engine and tranny I use Valvoline.
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I've used Valvoline GEAR OIL before and had no problems. I'm sure you probably know, but I feel compelled to say anyway.....
Do not use regular oil in the differential, It is way to thin.....Any auto store will carry gear oil....

I found out I can use Valvoline 75W90 Gear Oil for both front and rear differential but what do I use in the transfer case ?

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If you have Autotrac 4x4 I would highly recommend GMs fluid specifically for that (available from the dealer). If you have a floor shifter, I have no idea... I would enlist the help of the owner's manual (assuming you still have it) or ask the dealer what to use.

Anyone know how to find out what rear differential i have ?

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RPO code in glove box

I would use Royal Purple. You can go to www.royalpurple.com to find a dealer closest to you. It is an excelent oil. A little pricey but worth it.

I have to agree with horse, when i do mine im going with royal purple, im just trying to find somewhere that carries it. Ive seen dyno tests done to where the trucks performace increased after all the fluids had been changed.

What is this PRO code ? Only thing I see is the vin# Is there a way to see by looking at my rear axle to see wich one it is ? Only need toknow how much gear oil I need.
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I put amsoil in mine, and it seems to be doing ok. As for the transfercase, if you have the autotrac, automatic 4x4 button on the dash then you have to use the blue fluid from gm, its expensive.

I used Severgear 75 -80w in front, and 75-140w in the rear. I wish I would have just used the lighter stuff in both. I think it was $9 a quart.

Here is a site that you can look up the types of oil.

http://www.amsoil.com/scripts/runisa.dll?amsoiloaf:index

also I don't sell the product, I just use it. I have had very good results with it.