I'm looking to change the oil on the differentials of both my Subies: a 91 Loyale and a 08 Forester. Since this is Alaska and it's freeking cold, I am supposed to use either 80W or 75W/90 according to the owner's manual. Here's my dilemma: I already went to two auto shops, plus Walmart and my super market and they do not have either of those oil weights. I have found 70W/120 and 75W/140, the only two oil weights close to the recommended. Now, if I do get either of those two oils, would the lower operating temperature viscosity (120 or 140) be too thin for a differential? One in question, the 75W/140 is Royal Purple, the same brand I use for my Forester's engine, my oil of choice, and the bottle says that oil is formulated for limited slip diffs. I admit that despite me doing all my maintenance for the past 18 years and being a licensed aircraft mechanic I am a bit of a dunce when it comes to choosing an oil weight other than the recommended by the manufacturer.

I was able to find the correct wieght at Advanced Auto Parts. They even had it in synthetic!

i would continue shopping around, or order it in the mail if you have to.