C3 - 1971 - Recommendation for Rear End fluid

[Greg F.]

Guys,

I have a stock 350/270 hp. The rear end gear is 3:08. I'm going to change the rear end lube. I tried looking in the Archives but didn't find exactly what I was looking for.

What lube or lubes are recommended for my application? I saw one Thread that mentions GM Posi Trac Lube. I thought I saw another Thread a while back that describes the combination of 2 different types of lube. I'm not sure if this is needed for all situations or just when the rear has a "knocking problem".

What lubricant should I use? Your direction is greatly appreciated.

Thanks,
Greg

[Paul H.]

Re: C3 - 1971 - Recommendation for Rear End fluid

I have always used 80/90 weight gear oil and one 4 oz bottle of GM posi lube in my cars. Never had a rear end problem
Don't forget to do some figure eights in a parking lot to circulate the lube into the clutches.

[Gary R.]

Re: C3 - 1971 - Recommendation for Rear End fluid

If running a stock spring loaded posi then you may need 2 bottles of GM additive. Use only the GM stuff as well. For oil 90 wt will work but I use Lucas 85-140 in all my rebuilds and my own cars. I only use 1 bottle of additive with a tuned posi (springless).

[Duke W.]

Re: C3 - 1971 - Recommendation for Rear End fluid

Go to any auto parts store and buy two quarts of SAE 80W-90 GL-5 gear oil. This is a generic product based on an exact API specification. Brand name is of no consequence.

Buy two bottles of the GM Positraction additive. The part number should be easy to find in the archives, and any parts counter guy should know exactly what you want if you don't have the part number.

Mix one bottle of additive with the first quart of oil and use no more than half the second quart. Drive it around town for at least 30 minutes taking lots of turns and listen/feel for clutch chatter, and try to find an empty parking lot where you can do the "figure-eights" previously mentioned. If no chatter is evident (no unusual noise or jerking when turning a corner), fill the axle to just below the filler plug when the oil is warm/hot from driving.

If there is evidence of chatter, install half the second bottle and retest. If chatter persists, install the remainder of the second bottle. You only want sufficient additive to tame chatter because it reduces the effectiveness of the limited slip characteristics.

Once you have sufficient additive, top off with gear oil to just below the top of the filler plug when the oil is warm/hot from driving.

The axle oil capacity is approximately 3.7 pints.

Duke

[Greg F.]

Re: C3 - 1971 - Recommendation for Rear End fluid

Go to any auto parts store and buy two quarts of SAE 80W-90 GL-5 gear oil. This is a generic product based on an exact API specification. Brand name is of no consequence.

Buy two bottles of the GM Positraction additive. The part number should be easy to find in the archives, and any parts counter guy should know exactly what you want if you don't have the part number.

Mix one bottle of additive with the first quart of oil and use no more than half the second quart. Drive it around town for at least 30 minutes taking lots of turns and listen/feel for clutch chatter, and try to find an empty parking lot where you can do the "figure-eights" previously mentioned. If no chatter is evident (no unusual noise or jerking when turning a corner), fill the axle to just below the filler plug when the oil is warm/hot from driving.

If there is evidence of chatter, install half the second bottle and retest. If chatter persists, install the remainder of the second bottle. You only want sufficient additive to tame chatter because it reduces the effectiveness of the limited slip characteristics.

Once you have sufficient additive, top off with gear oil to just below the top of the filler plug when the oil is warm/hot from driving.

The axle oil capacity is approximately 3.7 pints.

Duke

Perfect! This is what I needed. Thanks Paul, Gary and Duke.