GV suggests Mopar synthetic gear oil, PN P04874459. So we traveled to Lakeland Dodge and picked up a quart. It lists for $21 a quart, but the guys sold it to us for $16. For the tranny, we used Royal Purple Max-Gear lube. Ya know, the Mopar synthetic gear lube is 75W85, and Royal Purple's is 75W90. Is there really much difference other than price? According to Gear Vendors, extensive testing has been done on the Mopar lube-their recommended oil.