As many of you know our 4speed manual transmissions and rear diff. should use GL4 oils, instead of GL4/GL5 ones.

I found this one. Just want to confirm with you that this is the proper one for our gears. Mine is 74, 4 Speed.

 

This is the one my trans builder said to use.


Others would void my warranty.

 

I use this. It's a GL-5.

http://www.redlineoil.com/product.aspx?pid=47&pcid=7

 

I use Redline GL-4 synthetic...not too many make a synthetic GL-4...

http://www.redlineoil.com/product.aspx?pid=46&pcid=7

 

Amsoil has a synthetic 75w-90 GL4 as well.

http://www.amsoil.com/shop/by-produc...?code=MTGQT-EA

 

My '71 owners manual states to use SAE 80 or 90 GL-5 gear lubricant in the 4 speed manual transmission.

 

My owner's manual and chassis manual also states to use 80w/90 GL5. I am going on 108,000 miles on the drive train.

As far as the rear. This thread is less than a year old and is an interesting read on a rebuild with pics Post #1839 shows the recommended fluid after the rebuild from an expert: https://www.corvetteforum.com/forums...-await-92.html

 

Supposed synthetic GL-5 is not good for the brass synchros...Reason I switched to synthetic Redline GL-4 a couple of years ago. I had run Mobil 1 synthetic GL-5 75W-90 for 25 years prior to the switch. To be honest, I don't think it will make any significant difference....change the fluid every 5-7 years and call it day.

 

GL-5 s are not good for yellow metals, they cause premature wear in transmissions only .. They are fine for Differentials

 

Yes that is the theory BUT for my 78 with the M21 4 speed like Karol ^ states above GM recommends GL-5, not GL-4, makes you wonder..

https://www.gmheritagecenter.com/doc...t-Corvette.pdf

Also, I did use Mobil 1 75W-90 synthetic GL-5 for 25 years in my 4 speed and there is no detectable synchro issues today..none.

Plus, there are MANY cars on the road with brass synchros in their manual transmissions that run GL-5 gear oil (since GL-4 is difficult to find especially as a synthetic) and I have checked many other automotive forums for any significant brass synchro issues with little evidence of an issue.

With all that said, I do use GL-4 synthetic Redline today but not so sure the GL-4 versus GL-5 controversy is real for the reasons stated above...This issue reminds me of the rear diff fluid controversy about synthetic versus conventional (and adding the GM addictive versus not with synthetics) and the engine oil debates (synthetic versus conventional AND which weight oil to run etc). I run, as an example, Mobil 1 0W-40 European Formula Group IV true synthetic engine oil in EVERY single engine in all my cars, power equipment (lawn tractor, snow blower, generator, etc) and NOT one engine manufacturer recommends that weight oil. AND every single one of these motors (some with LOTS of miles and hours) does NOT burn an oz of oil...makes you wonder about all these oil discussions...

 

I completely understand.. I too use Mobil One Synthetics in everything I own that requires oil..with the exception of my two stokes... have for decades with excellent results and never an issue.. I was simply stating the general wisdom on yellow metals reaction .. The theory is that as the GL5 coats the teeth and as it is worn off by time it takes more metal with it than GL 4 which does not bond as well and wears away easier.. thus causing premature wear on the trans.. Tests seem to bear this out to be true if you research it..

 

Would you possibly have a link to one of these specific GL5 tests?

 

Not handy, I saw them years ago.. I m sure You can Google it.. There is a definite coralation between the Yellow metals corrosion and the Sulfur content ,but I personally would use a GL5 Synthetic and not worry about it

 

So, I guess I could put same oil in the trans and rear end, just adding additive to the rear end is the difference.

If I'm not wrong proper combination should be:

- Dyno 85W90 GL-4 on the 4 speed Muncie trans
- Synthetic 80W90 GL-5 + 4oz GM posi add. on the rear end.

OMG, what a mess!!

 

Interesting that no one has mentioned what GM recommends.

That would be GM Synchromesh Fluid. I used that in my Muncie for years, and since Tremec recommends it for their TKO transmissions, I use it in my TKO600.

Also it gets high praises from the import crowd.

More info.........
http://www.bobistheoilguy.com/forums...Number=1114224

 

As far as the differential I have found the following to work extremely well, it keeps the differential from locking up and dragging one of the rear wheels when making a tight turn.

Lucas Heavy Duty High Performance 80W-140 Gear Oil (not synthetic)

Genuine GM Fluid 88900330 Limited Slip Axle Lubricant Additive - 4 oz. (A total of TWO bottles)

Do SLOW 20 tight figure-eights, then SLOW 5 tight left turns, and then SLOW 5 right turns. This allows for the gear oil and additive to well saturate the differential and it's clutch plates.

 

Many people stated that GL-5 oils, what GM recommend, are not good for our old muncies...maybe GM thought at that time that it was OK, and then experience told the opposite.


There are several variables:

- GL-4 or GL-5
- Dyno or Synth. oil
- 80w90, 80w140, 75w90.....

 

My '76 original owner's manual states GL-5, as others here have noted. But that is dino lubricant and not synthetic! I use Redline synthetic GL-5 in my 4-sp but it is a non-slip formula suitable for my transmission, hence, the designation 75W90 NS GL-5 gear oil. I have not used this brand in my differential. I am also not concerned that it will effect any metal types in my transmission although Redline does state it will not effect copper.

 

Using other oils cannot void the warranty according to the Magnuson-Moss Act. All the manufacturer can do is specify the parameters of the properties of the oil required, and any oils that are in spec can be used. It's a free trade kind of thing designed to protect consumers.

 

That's the issue, GL-5 synthetic or GL-4 dino?