69 corvette m-22 4spd oil rec

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Jan 1, 2013
South FL, USA
hey, i recently had my 4spd, m-22 rebuilt after having no 1st gear in my 1969 corvette. i really doubt the builder put the right spec fluid in there. i remember in the 80's we used either 75-90w [or thicker] gear oil or some applications called for the same weight that went in the motor, like a 40w. this car sees the road once every 3 months here in miami. the rear gear is 4.56 if that matters. i want to get a more street-able gear but the last owner installed it. so im here to listen to what the crowd says...happy saturday btw, about the rear end, what's the rec fluid for that guy too on here...
I used Redline MTL in my M21 equipped 1970 Judge for 15 years, best fluid experience for me and the car saw track duty as well. If you are uncomfortable with how light MTL is, Redline also has MT90. Both are dedicated GL4 oils so the shifting characteristics are spot on. My next GM muscle car I buy (very close to closing a deal) will also have Redline MTL in it within hours of ownership.
Use any GL4 rated 75w90 or 75w85 manual transmission fluid from Ford/GM/Mopar/Nissan/Hyundai/Kia or a boutique fluid like Amsoil MTG, Redline MT85, Redline MT90, Idemitsu MTF.
I've been using Amsoil manual transmission and transaxle fluid 75W-90 GL4 in my old Studebaker Hawk with a Borg Warner T10 4 speed. I like it better than the Redline MTL as the transmission shifts smoother and is a bit quieter with the Amsoil lube.
I've had best luck with a GL 4 80w90, like Sta-Lube. I had issues with synchros and seepage with the synthetics.
Back when I was running late 60s and 70s stuff, rear end was LS90 (Limited Slip 90 grade), and manuals of your type were 80W90m 75W90 if you could get it, and definitely GL4, GL5 was supposed to work, but never did properly on the shift to 2nd. Mobilgear XHP 75W90 is one of my faves. Mobil Fluid 424...has become my nearly all time fave 'though.
Im kind of Leary about using a modern syn fluid in these old trannys. I once used syn in a mopar 833 gearbox and it started to shift really slow because of the oil that clung to the syncronisers, a shift back to ordinary 80w-140 gl4 solved that problem. Modern syncros have been designed for the use of syns so nowdays it aint a problem for a tranny designed with syn in mind.
Originally Posted By: drfeelgood
I've had best luck with a GL 4 80w90, like Sta-Lube. I had issues with synchros and seepage with the synthetics.
Me too
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