Jamydodger

My manual says 80w90. I have seen a lot of people recommend the 75w140 synthetic instead. Which should i use in this old truck? it has 180,000 on it and i don't imagine the previous owner ever changed the oil. Would changing to synthetic hurt anything? Would synthetic 80w90 work ok? Newbie questions but i have no experience with diffs.

Thanks.

JCR 56

I would use what the manual says, it has worked good for a 180,000 miles so why change.