I would never run a 40 weight oil in a differential. I would never run anything less than 75w-90 in a gear set diff. 75w-140 is too thick, that is an oil to use in a large heavy truck towing weight. Due to the weight of the machine you may be able to get away with a lower weight in the rear but I wouldn't do it. engine oil I use would use rotella T6 5w-40. good for winter months and good for summer months full synthetic at a decent price.

75W-90 and 75W-140 have the same kinematic viscosity at 40C.

75W gear oil and 15W crankcase oil are essentially the same viscosity at 40C. The SAE grading scale is different for oils designated as crankcase versus gear oil.

I also think 15w oil is too thick for cold weather unless you have a 1955 Mac truck

I would never run a 40 weight oil in a differential. I would never run anything less than 75w-90 in a gear set diff. 75w-140 is too thick, that is an oil to use in a large heavy truck towing weight. Due to the weight of the machine you may be able to get away with a lower weight in the rear but I wouldn't do it. engine oil I use would use rotella T6 5w-40. good for winter months and good for summer months full synthetic at a decent price.

75W-90 and 75W-140 have the same kinematic viscosity at 40C.

75W gear oil and 15W crankcase oil are essentially the same viscosity at 40C. The SAE grading scale is different for oils designated as crankcase versus gear oil.

Yes the 75w oil has the same thickness at the cold temp, but it does not stay at that cold temp when you start to run the machine it heats up and thins out, the 140 does not thin out enough to provide good lubrication in my opinion, the 90 thins out more and is still thick enough.
I had a 95 taho which recommended 80w-90 oil, I had the rear end rebuild and the guy put 75w-140 synthetic when he did it, I got a horrible groan from the diff, this was because the clutch packs were not getting the lubrication penetration in them because the oil was too thick. changed back to 75w-90 and all good. You can argue oil weights all day, I go with what I'm comfortable with.

no argument here... just thought it could clear up confusions as I have heard from a lot of people who think a 75W gear oil is going to be like tar compare to a 15W crankcase oil

Yeah. It was 14k after doors and windshield and other things. I considered the new 1000 but its the first of its kind and bound to have issues. I'd rather stay with the tried and true 800. Its been proven time and time again

Synthetic 75w90, Works like a charm.