The '90 owner's manual says "MOPAR Gear Lubricant or equivalent 75W90 (API GL-5)" I think the consensus is GL-5 is incorrect and should be GL-4.
The Red Line MT-90 is 75W90 GL-4 and their site says:
"Safe for brass synchros, as it lacks the reactive sulfurs found in most GL-5 oils that cause damage" &
"Satisfies the gear oil viscosity requirements of 75W90, 80W90, 90 and motor oil viscosities of SAE 40, 10W40 and 15W40"