rear diff question

[srjd]

I service the rear diff every 50,000km I know its a little much but gives me piece of mind. Ive always used boss 75w140 synthetic with the friction modifier. Right on the rear diff it recommends 75-140 I was just wondering is this to thick for a canadian winter? Ive never seemed to have problems with it but i don't think id want to go with a thinner oil because of the lift and tires I have I feel like it wouldn't be good enough protection. Anyone want to add their two cents?