I'm almost at 100k on my '03 Chevy Express 3500. The gear oil in it is 80w-90. I want to change to full synthetic. The syns are 75w-90 or 75w-140. My local store has Royal Purple in both weights, Castrol Full Syn in 75w-140 and Lucas full syn in 75w-90.
Its a cargo van that gets used like one. It's mostly lighter loads, but occasionally its filled to the hilt with materials or towing some heavier loads. I also drive it pretty fast regularly, 80-90mph(if you lived in Chicago, you'd understand!).
My questions are..
1. What is the best full synthetic gear oil?
2. What weight should I use? I'm thinking the 75w-140 wt might be better?
I don't mind ordering online, plus I do have 3 other auto parts stores. I've heard good stuff about RP, but I do use M1 in the engine and I think one of the other stores carries M1 gear oil.
Its a cargo van that gets used like one. It's mostly lighter loads, but occasionally its filled to the hilt with materials or towing some heavier loads. I also drive it pretty fast regularly, 80-90mph(if you lived in Chicago, you'd understand!).
My questions are..
1. What is the best full synthetic gear oil?
2. What weight should I use? I'm thinking the 75w-140 wt might be better?
I don't mind ordering online, plus I do have 3 other auto parts stores. I've heard good stuff about RP, but I do use M1 in the engine and I think one of the other stores carries M1 gear oil.