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Discussion Starter · #1 · (Edited)
I'm going to change the differential cover on my newly acquired S10 since it seems to be leaking, probably because it rusted through. Since I'm removing the cover, I will need to change the gear oil in the differential.

Here is my question:
What oil should be used? I have searched (!) and found 80W-90 and 75W-140 in different threads (see below). Which weight gear oil should I get two quarts of?

Thanks,
Kevin

PS. 1998 2wd 4.3 auto with no G80 locker, FWIW.

80W-90 post 5
http://www.s10forum.com/forum/f13/rear-end-gear-oil-310231/#post4394128
75W-140 post 3
http://www.s10forum.com/forum/f13/gu6-rear-end-synthetic-oil-a-must-455027/#post7202329
 

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goin broke but n style
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either will work.
 

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Discussion Starter · #3 ·
OK, if one isn't preferable over the other, then I'll probably go with 80W-90 since it presumably has less viscosity modifiers in it.

Thank you for getting back to me.
 

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Gears and Differentials
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If this is just a driver then use the 80W90. If you will be using it to tow then use the 75W140.
 

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Discussion Starter · #5 ·
If this is just a driver then use the 80W90. If you will be using it to tow then use the 75W140.
Yep, it's just a daily driver; no towing or V8's on the other end of the drive shaft:eek:, so I'll put the 80W-90 in. Thanks.

Just out of curiosity, what is the tradeoff between the different weights of oil?

(I should probably just spend a few hours educating myself by reading BobIsTheOilGuy.com)
 

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i saw what you did there!
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it really depends on temperatures

temperatures affect viscosity alot

if youre gonna use the truck hard then the diff will probably get pretty warm so thicker fluid needed

but if the weather is really cold then you wont want too thick of oil either

so it depends..

but 85w90 should be fine

unless its super hot

i always use 85 90
never used anything else

dunno how 75 140 acts but when its insanely cold outside the 85 90 runs stringy really funny when its spilled etc.

like around 0 and below temps
 

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Discussion Starter · #7 ·
Yep, this will be driven in cold Wisconsin weather, but running unloaded most of the time.
 

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Gears and Differentials
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If the weather will be cold then a synthetic oil will work better. They flow easier at lower temps.

The difference between the 80W90 and 75W140 is the film strength of the oil. The higher viscosity usually has a higher film strength, which means that it's harder to push the oil out from between the two parts that are trying to make contact.
 
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