1980 K20 Rear Diff fuild

Nottle · Jul 10, 2024

Hey guys;

I have a 1980 K20 with a 14 bolt rear. The rear diff cover is leaking a bit, since the truck is new to me figured I would pull the cover and check her out.

The question is what type of gear oil are you guys running (weight and brand) and amount? From what I can tell it calls for 80w-90 and about 3 1/2 quarts.

If it matter I live in PA so it does get below zero sometimes in the winter.

legopnuematic · Jul 10, 2024

80-90, API GL-5 is good. Brand (in my opinion) is not too important. I’ve used mystic, pennzoil, valvoline, castrol, Lucas, harvest king, etc.

I’d recommend getting a 2 gallon jug, that way you have enough to service both front and rear differentials and should be a little cheaper than buying quarts.

SquareRoot · Jul 10, 2024

Verify the gear ratio while you're in there. Just trust me on this one.

Nottle · Jul 10, 2024

If i use standard Valvoline 80w-90 should I add a friction modifer?

Thanks in advance for the help guys. (and maybe girls)

Chris

legopnuematic said:
80-90, API GL-5 is good. Brand (in my opinion) is not too important. I’ve used mystic, pennzoil, valvoline, castrol, Lucas, harvest king, etc.

I’d recommend getting a 2 gallon jug, that way you have enough to service both front and rear differentials and should be a little cheaper than buying quarts.

legopnuematic · Jul 10, 2024

If you have a limited slip yes, if it’s an open differential no.

dsteelejr · Jul 10, 2024

I run 80-90 in my ‘80 K25 14BFF when it had an open diff and after installing a Detroit locker. I live in Wyoming and temps easily get to -30°F I in winter and haven’t had issues. Works fine.

Nottle · Jul 10, 2024

Looks like its a 14bolt FF G80.

Im going to grab a bottle of the Yukon Friction Modifier.

legopnuematic · Jul 10, 2024

If it is a limited slip, you may need two bottles of Yukon additive.

I believe they recommend 1 bottle for 2qts, over 2qts may require an additional bottle.

It may be fine with one, or clutches might chatter with only one.

1980 K20 Rear Diff fuild

Nottle

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legopnuematic

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SquareRoot

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Nottle

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legopnuematic

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dsteelejr

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Nottle

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