I have read the service manual on changing the differential oil and it says 15w40 or 80w90. To me there there is a vast difference between those 2 weights. Which one is the right one?

15w-40 is the engine oil. 80-90 is correct for the diff oil.

[2015 Z Force 800 53/Trail EPS]

You can run either but I suggest the gear oil in heavy, hot use and the lighter in the winter. I have other machines that require motor oil in the front diff and light tractor hydraulic oil in the read bit they have wet brakes and friction diff locks. I however split the difference by mixing some of my 10/40 Castrol Power-1 synthetic with my 75/90 synthetic. It did blend just fine.. and so far it likes it better then just the 75/90W. I run it in both.   

What about 75W140 full synthetic for the the huge temp spread we experience here in Canada.
Would that not cover all seasons?

I was thinking 10w30 during cold winter months for engine and maybe a mix of 15w40 and 80w90 for diffs in winter. I am going to change it as soon as first spring comes around. Lately is being in the 20s and 30s.

[2015 Z Force 800 53/Trail EPS]

What about 75W140 full synthetic for the the huge temp spread we experience here in Canada.
Would that not cover all seasons?

That's too heavy for any time of year. Remember, the carrier and pinion bearings get their lube from sling and splash..and that's not going to do that very well.

[2015 Z Force 800 53/Trail EPS]

I was thinking 10w30 during cold winter months for engine and maybe a mix of 15w40 and 80w90 for diffs in winter. I am going to change it as soon as first spring comes around. Lately is being in the 20s and 30s.

In the cold, you are always better off going with a lighter oil..even in the diffs. You will never hurt the diffs with 10 or 15/40 in any weather but...cold makes molasses out of gear lube that takes forever to warm up and start slinging around to the bearings. 

I would recommend changing out he 15w40 engine oil from the diffs straight away.
Run a gear oil for gears, the friction that is built from these diffs being so sloppy is amazing.
Run a good syn 75w90 or better is a must.

What about 75W140 full synthetic for the the huge temp spread we experience here in Canada.
Would that not cover all seasons?

That's too heavy for any time of year. Remember, the carrier and pinion bearings get their lube from sling and splash..and that's not going to do that very well.

Not quite an accurate statement. The 75W part is the lighter oil required for freezing temperatures. The 140 is for the extreme heat thus a large temperature range is covered.

https://www.mil-specproducts.com/Documents/1214_PDS27.pdf

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.

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.

Thanks for all the information. I am guessing 75w90 is what everybody is suggesting for differential. Probably go 80w90 in the Spring. From what everybody says 75  will be thinner cold than 80 and staying 90 on both will keep the hot the same. Looks like the 15w40 for engine oil. This is the first engine and differential oil change. I will change it again before Spring. It probably sounds overkill but you pay 14,000 for something you overkill everything anyway.

[2015 Z Force 800 53/Trail EPS]

I think for these it's sticking close to what they recommend and keeping it clean & full. After all, these aren't hundreds of HP machines, just 65.. and by the time it gets to the wheels more like 38. 14K!...wow. I think the new 1000 is 13,900.

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.

I think for these it's sticking close to what they recommend and keeping it clean & full. After all, these aren't hundreds of HP machines, just 65.. and by the time it gets to the wheels more like 38. 14K!...wow. I think the new 1000 is 13,900.

I think Canada gets the better warranty and better shocks out of the box though. USA gets the cheapness