Manual trans fluid ????

[Michael G.]

I will be changing out the fluid in my 71 manual transmission and have found only the synthetic gear oil at the dealership. It's got the GL-5 rating and is pretty pricey ($35.oo per qt.). Paragon has the GM 80-90W axle lubricant with an "EXCEEDS" GL-4 rating. I need some guidence here on what you guys use as the preferred fluid. I have read here about the new synthetics leaking past the seals. I don't beat on the car very often and put little mileage on per year. Please advise. Thanks, Mike

[Dick W.]

Re: Manual trans fluid ????

I would not go the synthetic route. As you mentioned, they will tend to leak quite a bit more. The sealing technology of the '70's left a little to be desired I would go to my local NAPA, Carquest, or store of choice and buy a reputable 80W-90, 85W-90 or what ever, as long as it meets GL-4 or better specs.

[Michael G.]

Re: Manual trans fluid ????

Thanks DW. I appreciate the assistance. Mike

[Peter M.]

Re: Manual trans fluid ????

I know that Dick is right about C3 transmission seals not being the greatest in the world. He is also right about the fact that synthetic gear oil can seep through old or weak seals.

HOWEVER, I can tell you that I have been using Royal Purple gear oil in my C4's Super T10 tranny for several years now and I have had no leaks at all. What's more important still is that the transmission has never been quieter or smoother. Finally, the Royal Purple gear oil has the positraction additive already in it so you do not need to add it. You will need a couple of pints at approximately $13 each to fill your transmission. http://www.royalpurple.com/gear-oil.html

The bottom line, in my view, is that if synthetic lubricants cause your engine, transmission or differential to leak, then FIX THE LEAKS.

FWIW

[Jephrey S.]

Re: Manual trans fluid ????

I tried synthetic gear oil in my M21 and promptly changed back to pure dino 80W-90 due to the leaks. If you have all new seals the synthetic might make more sense.

As far as the pre-added posi-lubricant goes, I have not tried the Royal Purple in my differential but I did try the Castrol semi-synthetic that claimed to have the pre-added lubricant. It still took 2 bottles of the GM additive to shut up the posi complaints from the pumpkin.

[Peter M.]

Re: Manual trans fluid ????

I did try the Castrol semi-synthetic that claimed to have the pre-added lubricant. It still took 2 bottles of the GM additive to shut up the posi complaints from the pumpkin.

All I can tell you, Jephrey, is that I have been using Royal Purple gear oil in both my tranny & differential for several years now and I have experienced no leaks and no abnormal noise. What's more, the transmission is much quieter and shifts much smoother. Also, my transmission used to pop out of second gear when coasting down a hill (probably due to worn syncronizers) and after refilling with Royal Purple it no longer pops out of gear. Of course, this is my personal experience, others may not get the same results.

P.

[Dick W.]

Re: Manual trans fluid ????

I know that Dick is right about C3 transmission seals not being the greatest in the world. He is also right about the fact that synthetic gear oil can seep through old or weak seals.

HOWEVER, I can tell you that I have been using Royal Purple gear oil in my C4's Super T10 tranny for several years now and I have had no leaks at all. What's more important still is that the transmission has never been quieter or smoother. Finally, the Royal Purple gear oil has the positraction additive already in it so you do not need to add it. You will need a couple of pints at approximately $13 each to fill your transmission. http://www.royalpurple.com/gear-oil.html

The bottom line, in my view, is that if synthetic lubricants cause your engine, transmission or differential to leak, then FIX THE LEAKS.

FWIW

Peter, some of the earlier design sealing technologies are almost impossible to keep from leaking with conventional oil, let alone synthetic.

[Duke W.]

Re: Manual trans fluid ????

What Dick says. Just go to any auto parts store and by two quarts of any brand 80W-90 GL-5, and it shouldn't cost more than about three bucks a quart. That's what GM used for plant fill and recommended for service. If it works, dont fix it!

IIRC the capcity is about 2.5 pints, but check your owners manual.

Duke

[Michael G.]

Re: Manual trans fluid ????

Funny thing Duke, I was loking through the owners manual and the shop manual and could not find a definitive fluid capacity for the manual trans. I last changed a 4 speed's fluid back in the 70s and didn't want to rely on my fading memory. My 71 drank 1-1/2 quarts on level ground. Thanks for all reponses. Mike

[Dennis S.]

Re: Manual trans fluid ????

Or you could really splurge and try Lubrication Engineers Monolec. I did and paid around $13.85 per quart. After spending $800 rebuilding the M21, and considering I will probably never change it again, what the heck.

http://www.le-inc.com/products.jsp?productID=80

[Duke W.]

Re: Manual trans fluid ????

It's a good idea to change manual transmission oil - say every 30K miles or at least every 60K.

Manual transmission oil usually doesn't run hot enough in normal driving to see signficant oxidation, but it does become contaminated with tiny brass particles from synchorizer wear.

Duke

[Alexander C.]

Re: Manual trans fluid ????

Or better yet Redline MT-90

[Dick W.]

Re: Manual trans fluid ????

In a former life, looking after the maintenance of over 2,000 vehicles, we never saw any advantage of anything but garden variety GL-4 or GL-5. We did see better shifting with 50-W motor oil in certain applications, but I would not recommend this for a Corvette

[Frank E.]

Re: Manual trans fluid ????

I just did my '66 and used Penzoil's "syncromesh" GL. On the Muncie, it takes about 40-45 oz. so buy two quarts. Cost you about $16. I was having a 3-4 slight grind and after changing the grease, no more grind

Frank