StrayDog

Draind trans fliud from my 4 speed, (1966 big/block) I drilled a 9/16'' hole in boss on side of transmission , drined fluid & tapped & installed drain fitting plug, .. :smash: thinking of going with a synthetic :confused: any suggestions............... :seeya :steering: :auto: :flag

SWCDuke

The OEM spec is MIL-L-2105. This is the same as current API 80W-90 GL5 gear oil. You can buy this in mineral or synthetic base, but be sure you buy a brand that DOES NOT have the limited slip friction modifier. Most don't, but Redline, for example, offers both. The one you want for the transmission is called "75W-90 NS gear oil". Their " 75W-90 gear oil" DOES have the limited slip friction modifier additive.

I don't think synthetic will buy you much, especially if you don't drive the car in cold weather. I would recommend standard mineral oil based GL-5 and change it every 30K miles, or more often if you are hard on the transmission. Periodic manual transmission fluid changes will flush out most of the tiny brass particles that inevitably build up due to synchonizer wear.

Duke

JoesC5

Duke,
I was under the impression that GL4 was better suited to transmissions and GL5 for rear ends(but can also be used in transmissions). Am I misinformed?

StrayDog

Duke>>>>In my situation , I do drive in cold weather ( or much colder than west coast/Calif)in the fall / spring, many short drives (20 miles)in 40 deg weather.............So I want to go with new tech on trans fluid( I use Moble one 10/30 ) in my 427, & very satisfied ............... The Dog (cold dog) :crazy: :steering: :auto: :flag

SWCDuke

I depends on the transmission. Most modern manual transmissions or transaxles without a hypoid gearsets (such as used in FWD cars) typically specify ATF or a GL-4 with GL-5s reserved for axles with hypoid gearsets, but the original spec for our vintage four speeds was GL-5. The Mil-L-2105 spec applied to both the manual transmissions and axle in C2s.

GL-5s have extreme pressure additives than modern synchronizers don't like. Synchronizers need some friction to operate properly, and GL-5s can be "too slippery" for them, but GL-5 is the recommended lubricant for our vintage three and four speed manual transmissions.

What would be the effect of running a GL-4? I don't know. I've never tried it. There may or may not be a noticeable change in synchro action, and any long term effects would require a disciplined long term test, which most of use aren't set up to do.

The C5 is generally known as having a "transaxle" but that is a misnomer. The transmission and axle are bolted together, but have separate housings and lubricant supplies. I think the six-speed manual lubricant is ATF and the axle uses a GL-5. Transaxles that combine both a manual synchronized transmission with a hypoid gearset are problematic when it comes to lubricants due to the different and often conflicting requirements of synchronizers and hypoid gearsets. Porsches, Audis, and the ZF transaxles used in the vintage GT-40 and Pantera come to mind, and GT-40 and Pantera owners are advised to use the special transaxle lubricant sold by Porsche/Audi dealers.

Duke

hot1corvette1

I was having problems with my Muncie. Put Redline NS in after draining it completely. The drained fluid was very black and had a lot of small brass particles. Runs like new....very smooth. Actually easier than my C5.

DansYellow66

I run synthetic oil in my muncie with no problems. :seeya

CO_Sledgehammer

I run the Redline 75W90 NS in my M20 w/ Hurst and it's smooth as can be.

magicv8

I use the old 80-90W mineral based gear oil, and the Muncie will stand all the power shifts that I can throw at it.

Do any of the 4 speeds with other than 80-90W mineral based gear oil leak?
:skep:

JohnZ

I guess I'm just an old-fashioned engineer - I run GM diff lube and GM Posi additive in my diffs, and good old 80W-90W gear lube in my Muncies, and have in many cars over many years, never had a problem (or a leak). :thumbs: