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Discussion Starter #1 (Edited)
I recently picked up an 88 Suburban 3/4 Ton and am working through the fluid changes. I picked up a FSM but I have a few questions.

Specs: 14 bolt 9 1/2" Rear Axel - 8lug - limited slip
10 Bolt 8 1/2" Front Axel - 8lug
TH-400 Auto Tranny

I used Chuck's Chevy Page to take a look at the RPO codes and determined that it has a limited slip rear diff. I confirmed this in FSM so I am confident in that.

The maintenance guide that came with the truck says to use 80w-90 GL5 for the front and rear diffs. The FSM says use use 80w-90 GL5 for the standard diff and 80w-90 for the locking diff. It also says if you are in Canada to use 80w GL5 for the locking diff and lists the same part # for both.

Based on what I am reading, you use the same gear oil in either one. Is that correct? Or am I missing something?
 

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its incorrect. the oil for the posi isn't the same, and there is also a friction modifier that should be added when changing out oil in an older posi.
check the dealer for the correct fluid and a bottle of modifier.
 

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Discussion Starter #3
I was planning on using Red-Line 75W90 which has the friction modifier in it as well. When I spoke to Redline that is what they recommended.
 

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The '88 has a gov-lock, not a posi. The friction modifier should not be used. Any normal gear oil will work fine front and rear.
 

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Discussion Starter #5
Tim,
Redline also has the 75w-90 in a NS version without the friction modifiers. Is that what I should be looking at?
 

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That will work great.
 

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Discussion Starter #7
Tim, thanks for the info. After searching through the FSM I was able to locate where it says not to use a friction modifier. I wish they put that with the fluid recommendations to make it easier. However, I guess when this was written, friction modifiers probably did not come already in the fluid so you did not add it unless it said to do so.

Thanks again for your help.
 
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