With hy-tran, there can be prolific gear failure, especially when using the tractor under heavy load. The big MM engine has an incredible amount of torque for straight cut gears that were already made of questionable material. You could maybe get by without a heavy oil if under extra light use, but from my experience, it takes one big pull to really start gear failure. Plus heavy oil will quiet the noise from straight cut gearing.
The old torque should be checked with the second, more forward plug on the torque compartment. It is slightly higher to ensure torque lubrication. Without an oil cooler, the front pinion bearing is capped to keep oil from leaving (and sometimes from getting into) the torque compartment.