So I'm about to change out the fluid in my differential, got a bit of the crunchies on slow sharp turns after the rearend has warmed up a bit. Time to suck out the old stuff and add two bottles of GM Posi additive, I'm all good with that. The issue I've come across is the whole GL4 vs. GL5 rated gear lube. Not finding much specific to the differential, but there's all kinds of info out there about NOT running GL5 fluids in your Muncie 4 speed. Changed my fluid out last year and sure enough the bottle from Autozone was GL5. This whole thing reminds me of the oil w/o ZZDP !! So did some research and found GL4 rated gear lube by the gallon at Napa, thought I was all good to go on both the tranny and diff., UNTIL I read my 73 GM Passenger Car Service manual and it's got GL5 noted everywhere as the reccomended gear lube , really !!?? So I decided to come to the source of everything C3. Did a search on GL4/GL5 and didn't get any hits. Anybody familiar with this issue ?? Sarge, Joel, anybody ??

|UPDATED|6/25/2015 10:06:58 AM (AZT)|/UPDATED|


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John Sigmund

Go with what the Service Manual recommends. As long as it is not a synthetic lube, you're good to go.

 There was an article recently by one of the NCRS guys that actually researched the differences in the lubes, and he laid out the pros/cons of each, and why the factory recommended this or that lube, and also compared what is available now with what was available back when the cars were new(er). The specs for the lubes haven't change, but the formulations have.

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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Thanks Joel, I'll check out the NCRS post. The inter webs are a great thing, but sometimes a bit of information overload !!

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John Sigmund

  PENZOIL makes a great lube for older yellow metals. I just changed mine, and it made a LARGE difference in the shifting ease. Yellow bottle, 75-85w. $7.95 a qt. Well worth the cost.

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Thanks MS, I'll keep that suggestion in mind. No shifting problems here, I just wanted to make sure GL-5 was not going to cause any problems. 

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John Sigmund

Interesting read...... http://www.widman.biz/uploads/Transaxle_oil.pdf

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John - so what are you running in your diff. and 4 speed ??

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John Sigmund

I just put Valvolines Synthetic 75w80 in my rear and M21 .  Nice shifting . No problems.    I had a drain hole put in the bottom of the Diff.  and it made the task so much more enjoyable and simple. 

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1970 built 454 coupe., M21, 336-1 rear, numbers matching . Approx. 550 hp/600 tq. #2 Condition .
For Sale :  $32,000 very firm .
Have a full write up with closeup walk-around video for the serious minded only please.