I've been looking at various gear oils to replace the factory fill for -35 to -40C winter use in the differentials on an 07 Nissan Pathfinder. The manual states 80w-90 'Super Hypoid' GL-5 use for the front diff and 75w-90 GL-5 for the rear, but I find that there are large viscosity variations between brands and many 80w-90's don't have low enough pour points for extreme winter temperatures.
Local Canadian Nissan dealerships change the front and rear diffs to Mobil 1 75w-90 for winter use which has 40/100C visc numbers of 106/15.2 cSt and a pour point of -46C, but that oil is for limited slip differentials and this PF doesn't have LSD's so I don't think I need to have that extra friction oil.
My understanding of gear oils is that to be an SAE XXw-90 the oil has to have an 100C viscosity of something above 13.5 cSt but less than 18.5 cSt, that to be considered to be a '75W' the oil needs to have a pour point of close to -40C, that low friction oil doesn't work in LSD's because it doesn't have enough friction to keep the components in place for function and that a 'non-shearing' gear oil stays in grade better than an oil that shears, but please correct me if that info is wrong.
A couple of low friction options that look interesting are Motul Gear 300 with a 100C of 14.2 and Shell Helix RGO 75w-90 with 100C of 14.9 cSt, Motul further states that it is not for LSD's but I'd be interested to hear of other gear oils that will stay within the 75w-90 requirement, are 'extreme load' GL-5 oils and also are of a low friction type, not recommended for LSD's. Thanks
FWIW ... on my 1989 4WD Toyota Van (yes, they made a 4x4 "van") I could not move the vehicle at -40 with the standard weight diff oil (75-90). I changed to a Shell synthetic similar to the Helix (can't remember which one exactly ... it's been a few years!). I could start and (after a 5 min warmup for the engine) drive at -50 with no hesitation ... saved my butt (and others whose cars refused to move) on a number of very cold days.
After 430,000 km I sold the van AND just because I was interested, I checked the diffs ... oil like new ... no noticeable difference in the viscosity or the "look" (opacity, color, etc.). It's not scientific but it worked for me.
Welcome to the site GreyBrick, personally I'd run 75w-90 in both your front and rear diffs in winter, and depending on your usage a 75w-140 come summer if you tow or offroad. To my knowledge, 95% of GL5 lubes come with a friction modifier in there. The amount required for a clutch based LSD is are 4 oz or so, so the amount present should be negligible considering most diffs take 2 or more quarts of oil.
Given that PAG type gear lubricants usually have considerably lower friction coefficients than PAO and Ester/Diester type oils, I see that there are a few companies that offer PAG's for various uses, such as this industrial PAG that aproximates 75W-140;
Igol France makes an industrial stabilized PAG which claims extreme pressure use for hypoid and other gear uses, reduction of friction coefficient and is miscible with mineral oils, look for the Synarok GY and Synarok Synth. products about a quarter way down the page;
...To my knowledge, 95% of GL5 lubes come with a friction modifier...
Here's another interesting PAO gear oil option, 75w-90NS from Redline which is a GL-5 but with low viscosity and pour point numbers and to which you have to add 'friction modifier' to use it with most limited slip diffs. Interesting to see that the 'NS' has lower viscosity numbers than the RL regular 75w-90.
I found one North American PAG type automotive gear oil manufactured by Dow Chemical that looks interesting in that it has a road use study in tractors to back up fuel economy claims. The ISO 150 would be about an 80w-140 grade but I suppose mixing an ISO 68 and 150 would get to about a 75w-90 equivalent grade. Wonder if the oil is equivalent to or exceeds the API GL-5 extreme pressure, pour point looks good at about -40C. Not sure if the 'Coefficient of friction' was obtained using a standard ASTM test but if so the 0.0184 number is about half of most petroleum or ester based synthetic oils.