Originally Posted by Beaver5
I would go with the MTL. The other is like 75w 90 gear oil without the stuff that is bad for your syncros. The owners manual is wrong and if you dont think it is call a jeep dealer and ask what the part # is for the ax15 tranny fluid and when looked up turns out its 30 weight motor oil. The MTL is about the same as 30 weight motor oil but made for manual trannys. Shifting is better in several guys jeeps I know with MTL than thicker fluid. Im running stait 30 rotella in mine right now but am going to change it in another 20000 miles. Theres a jeep shop not to far from my house and the machanic there told me the same thing.
Yea... kinda dealerships seem to have different philosophies about exactly what to put in the ax15. Some people use 10w30 and claim its the best, most use 75w90 GL4/5. You want to use 75w90 GL4 (gl5 has the phosphates that react with certain synchros) 10w30 works too, the dealerships here use 75w90, elsewhere I've heard of 10w30, but overall all of the documentation I've seen says 75w90.
The main difference between 75w90 and 10w30 is the viscosity. The reason you use 10w30 in an engine is because the thinner lighter 10w30 is moved around the engine parts through an active pumping system inside the engine. Differentials and manual transmissions do not have similar systems. Instead they rely on a passive system of a large oil sump at the bottom of the pan which the gears scoop up on the bottom and carry up with them over the top of the gears and then the oil trickles down into the gears. A lighter fluid like 10w30 isnt suited as well for this task as the thicker 75w90, on account that you need a thicker fluid to be able to be carried from the sump to the top of the gears. Now 10w30 works, 75w90 was made for this.