F23 Transmission Rattle

Rauq

Goblin Guru
ZZP recommends Redline MT-90 for the F35. most of what I've read says the F23 doesn't really care, but most say ATF over gear oil (which is thicker than ATF). I did find someone talking about overfilling with a specific gear oil on a J-body circle track application, but I can't find it again.
 

Rauq

Goblin Guru
My '06 Cobalt owners manual says DexIII for the F23. DexVI is an approved replacement for all DexIII applications. I can't say 75W is wrong or needs to be replaced or that ATF will fix a rattle, but if you want to try it that doesn't seem like a bad idea to me. If you're going that route, do try 2.6qts through the vent plug on the top instead of 1.8qts through the side fill plug.
 

Dsteinhorst

Well-Known Member
Couple thoughts to add to this.
I consider Pennzoil synchromesh a good fluid and have used on several transmissions over time. It helped wake up my notchy F23.

If you go above factory fill level, you may get leakage out the vent during hard cornering. I lost fluid at a couple autocrosses until I drained to the standard fill line. I had maybe an extra pint over factory.
 
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