Trying to find articles on GL4 vs GL5, GL5 is corrosive to synchros and more of an extreme pressure lube for hypoids and can hinder synchro performance. Remember, a synchro is like a brake. As you slide the shifter from one gear to another, you apply pressure on the synchro (cone shape) which then using friction, gets the mating gear set rotating at the same speed and once the speed is matched, the hub slides over the mating spline to lock that gear to the shaft.
Our transmission are constant mesh design, meaning gears are always meshed already, we are just locking the gears to the shaft. That is done with the synchro and another cool device called an inertia lock. Synchro matches the speeds, inertia lock holds the gears in place. The inertia is what some call dog teeth, the same teeth are on the underside of the slider. Pointed tips, and the sides of the teeth narrows as you go to the base of the teeth. This narrowing is what holds the gear in place under a load. When you grind a gear on a full synchro transmission, you are clashing the dog teeth. There is a drive side and a coast side on these teeth. If you ever get a gear that pops out under load or decel, the teeth are narrower at the tip instead of the base one either coast or drive side, so under load, they push themselves apart. The dancing shifter during accel and decel is a sign dog teeth is going away.
Certainly GL5 temporarily isnt going to kill your transmission, but if it is GL5, over time, synchros will corrode (eats yellow metals) and the extra friction modifiers in GL5 impede the action of the synchros. Synchros rely on some friction, GL5 makes them slipperier hindering shifts.
OK, probably more info than you needed, but good info for other members.
My truck with my 75W90 GL4 is only clumsy cold shifting down into second which is my main start gear, I just come to full stop before shifting to second. Shifting up isnt an issue and in short order, all is good rest of the day, shifts fine after 20 minutes or so.
You installers said gear oil, that is a generic term for both hypoid and transmission oil, but to refine it more, hypoid or transmission oil. So its possible they did put in GL4 but you mentioned stink, that usually points to GL5
Here is a link, any generic search using GL4 VS GL5 should get you some more answers.
GL4 vs GL5
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