I just had my Saginaw M20 tranny rebuilt. New seals, bearings, synchros, 3rd/4th slider. I currently don't have any gear oil in it at the moment, and I was wondering if you guys could give me some opinions on going synth vs. natural gear oil when I fill this thing. Any information would be appreciated... thanks!

 

Hey Stale, I havent made up my mind yet on tranny oil but I will give you some ideas to ponder. First off, gear oil has an EP additive (extreme pressure) so that the oil doesnt squirt out from the between the gears under load. This is a needed additive however, the EP additive is mildly corrosive to soft metals (things like your synchros). This is the main reason why I switched to synthetic as some synthetics use a different EP additive which is ok for soft metals. When I did the dump and fill with synthetic and fired it up I could feel the shifter moving smoother and easier the first few times I moved it through the H pattern at a stop. That impressed me. NOw we all know that synth is a smotther, more slippery product which is what makes it better for loads. But now back to the synchros. They are a friction material, basically a clutch to slow the gear down enough to engage the shift sleeve w/o grinding. I have found my syncros to work a bit less effectively after I went to synth. So this is where I stand on it. As of now I am running synthetic, but I may go back to dino gear oil for the extra grip of the syncros.
Just some food for thought. If anyone else has some input I will be happy to hear it.
CoryM