shown here is the transfer case of a suzuki jimny/samurai. simple TC w/ old-school mechanical shift lever. all the power goes through the chain (green) even the rear wheels so yes you're on the chain while you're towing a heavy trailer on 2-HI
factory fill is a super thin 75w GL-4, though the previous generations of the jimny had 75w-90
if fuel efficiency doesn't matter, and minimizing wear on the transfer chain is the priority, would thicker oil be better? thicker than 75w-90? or maybe even a dual-rated GL4/GL5?
