M21 Correct Oil

[bcmiller]

Steve, this is a quote from Red Line.

GEAR AND SYNCHRONIZER WEAR PROTECTION
Most manufacturers of manual transmissions and transaxles recommend an 80W or 90W GL-4 lubricant.

GL-5 gears oils which are required in hypoid differentials are not used in most synchromesh transmissions because the chemicals used to provide the extreme pressure protection can be corrosive to synchronizers, which are commonly made of brass or bronze. Typically, the use of a GL-5 lubricant in a synchromesh transmission will shorten the synchronizer life by one half. The extreme pressure requirements of spur gears and helical gears found in transmissions are not nearly as great as found in rear-wheel drive differentials.

A GL-4 lubricant provides adequate protection for most manual transmissions, unless a unique design consideration requires the extra protection of a GL-5.

Now, Paul C. has rebuilt a lot of Muncies and has not seen any issues BUT, why take the risk?  See this link for his comments.

http://www.chevelles.com/forums/showpost.php?p=7482682&postcount=66

[L78 steve]

Steve, this is a quote from Red Line.

GEAR AND SYNCHRONIZER WEAR PROTECTION
Most manufacturers of manual transmissions and transaxles recommend an 80W or 90W GL-4 lubricant.

GL-5 gears oils which are required in hypoid differentials are not used in most synchromesh transmissions because the chemicals used to provide the extreme pressure protection can be corrosive to synchronizers, which are commonly made of brass or bronze. Typically, the use of a GL-5 lubricant in a synchromesh transmission will shorten the synchronizer life by one half. The extreme pressure requirements of spur gears and helical gears found in transmissions are not nearly as great as found in rear-wheel drive differentials.

A GL-4 lubricant provides adequate protection for most manual transmissions, unless a unique design consideration requires the extra protection of a GL-5.

Now, Paul C. has rebuilt a lot of Muncies and has not seen any issues BUT, why take the risk?  See this link for his comments.

http://www.chevelles.com/forums/showpost.php?p=7482682&postcount=66

Good info. Thanks for link to the trans expert.

[jeff68]

Sorry I'm a little late to the game on this one, but I've had to deal with the issue of yellow metal corrosion and gear lubes at work. 

The issue really isn't GL-4 vs. GL-5.  Some people claim that the GL-5 gear oils attacks yellow metals, but most gear oil manufacturers now use non-active sulfur phosphorus EP (extreme pressure) additives which don't attack the yellow metals.  Also, even if it is the 'old' type additive, yellow metal attack usually doesn't occur until the temperature gets to about 240F - hopefully above the temp that any of your internal gearbox components experience.

The problem with finding a gear oil for use in transmissions is finding one without Limited Slip (LS) additives. My personal experience is that the LS additives are usually what gives you problems with your synchros and causes grinding upon shifting.

So, if you can find a GL-4 or GL-5 that doesn't state that it has LS additives, then you should still be OK using it in your transmission. I have yet to find any GL-5 locally that does not contain LS additives. I used to be able to find Exxon Superflo 80W80 GL-5 without LS additives, but I believe it is no longer available.

I use CRC StaLube GL-4 85W90 gear oil in my M20.

[miket1]

I bought  Brad Penn  Classic  GL4, 80w90,  made for older trans. with Copper, Brass, Bronze  synchronizers, it was easy to find at Summit.