Big truck transmissions have no synchronizers and no yellow metal. However, with the higher torque ratings of electronic engines which came of age 20 years ago, transmissions were developed to handle them. I am in the Texas hill country and I set my engines at 1450 ft. lbs. torque. However they are capable of putting out 1850 ft. lbs of torque. The MT-1 is fully synthetic and it is pours almost like Mobil 1 15w50. However, it does not get much thinner when it is hot. Our transmissions all have pumps and an oil cooler in front of the radiator. When we still ran mechanically injected engines, we ran 90wt straight mineral oil in them. GL5 would make them very difficult to shift. I don't think it hurt anything, but the drivers would sure complain if someone put in GL5 by mistake. I am not familiar with 600 wt oil nor 160 wt. We used to run 140 wt GL 5 in our rear ends in the summer and 90 in the winter. We now run 80w140 fully synthetic and never change it. I run 50 wt fully synthetic in some of my old jalopy transmissions. I don't think it hurts anything, but it is really too slick for the synchronizers to work properly. That doesn't bother me because I am accustomed to shifting without a clutch any way.
