The correct lube for a Jeep synchronized transmission
Be sure when you install lube in the transmission/transfer case, that it is for synchronized transmissions. No EP additives should be in it. They (EP additives) have a few bad effects on synchronizer's, especially brass ones. I cannot tell you how many times someone put standard 90wt or 140wt gear oil into a bronze synchronized transmission, only to complain that now it is miserable to shift. Once its in there for a while, it usually effects the synchro's for good.
First and foremost the common UN-spoken concept of the synchro, is not discussed in the "Lube" conversation. The synchro in the T90 is a brass cone clutch. As the shift collar is moved it engages the teeth on the synchro and the inside of the synchro (the cone) presses against the machined surface of the next gear, this pressure spins the next gear up to speed and the shift is "synchronized". Any EP additive impairs the friction caused by the pressure of the cone pressing against the gear to spin it up. This just screws up the shift.
Next is the incompatibility of EP additives and brass synchro's. Most EP additives are corrosive to brass. There is a You Tube Video out there that is not quite accurate so, be careful! There is however some pretty good stuff put out by the manufactures. As always, dropping a line to the Tech Department at your preferred supplier is the way to go if you want to be sure of what you are buying. If they can't answer your questions with fact, buy elsewhere.
The EP additive IS REQUIRED in the differential however!.
The real trick here is to match the EP ability of the oil/lube to the application. Here it is important to remember to much EP capability (as intended for use in sliding hypoid applications), inhibits the synchro's ability to engage the next gear and "clutch". This causes more gear clash than normal and wears the clutching gear teeth faster. In addition the shift will be "stiffer" and slower than it was designed. The brass facings will become glazed making things worse. Most will never hear the excessive gear clash, there is usually to much other noise. This damage is subtle.
GL-4 gear oil does have some EP. It is not so excessive that it causes shift issues. GL-5 is tops on the EP additives list, and is intended for the high sliding friction of a Hypoid gear set. The reverse error of putting a low EP in a Hypoid gear set is obvious and happens quickly. It is a total failure. The failure analysis in the Tech Section has some good pic's. If you make a mistake and use a high EP in a cone style synchro, life isn't over. Many have done it. Some don't notice. A sharp transmission re-builder familiar with old school synchro's, would know immediately.
There is another subject about the difference in the pour-ability of different types of lube like GL4 vs GL5. Gl5 pours significantly thicker at the same weight.
Here are two good examples of great old school synchronized transmission lubes.
https://www.redlineoil.com/mt-90-75w90-gl-4-gear-oil
https://penngrade1.com/products/gear...il-sae-80w-90/
