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Rear Oil Viscosity Question

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Old Nov 4, 2015 | 01:58 PM
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Default Rear Oil Viscosity Question
Along with the posi additive what is a correct oil weight to use in the differential oil change?
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Old Nov 4, 2015 | 02:34 PM
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I've always just used 80/90 Gear Lube....same as the trans.
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Old Nov 4, 2015 | 05:25 PM
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Originally Posted by gungatim
I've always just used 80/90 Gear Lube....same as the trans.
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Old Nov 4, 2015 | 07:15 PM
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For most Vettes I use this when i rebuild or change the rear diff oil.

http://www.valvoline.com/our-product...hetic-gear-oil

For higher power cars running gears like 4.11 I use the synthetic 75W-140
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Old Nov 4, 2015 | 07:36 PM
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Originally Posted by gkull
For most Vettes I use this when i rebuild or change the rear diff oil.

http://www.valvoline.com/our-product...hetic-gear-oil

For higher power cars running gears like 4.11 I use the synthetic 75W-140
Why the big difference in viscosity from 80/90? I thought it was not recommended to mix conventional and synthetic oils.
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Old Nov 5, 2015 | 12:59 AM
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Originally Posted by Vettesic
Why the big difference in viscosity from 80/90? I thought it was not recommended to mix conventional and synthetic oils.
Higher numeric gear ratios create lots of heat. My 4.11 would get so hot that you could not touch it after a 20 minute track session. It really needed a pump and cooler even with the 75W-140. My richmond gear sets still looked new after many years of use. Synthetics have some good qualities

I didn't mix with conventional
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Old Nov 5, 2015 | 07:04 AM
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75W-90 Mobil 1 synthetic for street driven C3's, no additive needed. Been using it since the early 90's with zero issues. I have been using synthetic oils in trans, diff, and engine oil for many years now...don't use conventional oil if synthetics are available for the application since a synthetic oil is superior to conventional oil in most applications in every aspect.
Last edited by jb78L-82; Nov 5, 2015 at 07:07 AM.
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Old Nov 5, 2015 | 08:26 AM
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I've been using 80/90 gear weight oil (non synthetic) in all of my older Corvettes since the early 70's with one 4 ounce bottle of GM posi lube. Never had a rear end failure. My 65 still has the original rear end in it. Never been rebuilt. I've owned it since 1971 and have driven it over 250,000 miles. They were designed for these fluids. I like synthetics in newer Corvettes but it's hard to mess with success in the older cars.
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Old Nov 5, 2015 | 08:39 AM
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IMHO,
I'm with CapeVettes on this 1.
I have a stock '68 BB and use the rec. orig. oil weights in all
components. She has 84,000 orig miles now and no issues.
The tolerances on the older big blocks were relaxed when compared to the newer tighter engines.
An older engine rebuilt to tighter specs than I would utilize the latest oils.
Marshal
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Old Nov 5, 2015 | 09:01 AM
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I used Lucas 75-140 in my diff. It was the recommended oil by Tom of toms differentials and also Gary Ramedi.
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Old Nov 5, 2015 | 09:56 AM
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I use 85-140 because I have to pull a 10 mile long 6% grade every day. Heat wise its like running 150 mph for 15 minutes...................
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