Nope, nothing to worry about. The 75w will actually flow better in the cold and if it's synthetic like what you're using, will protect better in the heat as well.

 

Nissan VISCOUS LSD are sealed units so both diffs use the same fluid type.
The highest friction of any differential occurs at the slowest speed and decreases as speed increases........under 10 mph is very high and decent by 55 mph.

Because the fluid is so thick cold and so thin hot........finding a viscosity balance is critical unless you are always in an even warmish climate with little winter.

Measure your summer peak and average diff temp after [during] a 2 hour drive at highway speed.........even a 2% friction loss creates 1 HP of heat ~~ 12,000 BTU [that electric home space heater times 4]

 

I live in NC...rarely gets below 20 here, average temp 74...90's in summer

 

can I pick this stuff up at the auto store?

 

You can find a local Redline dealer here: Dealers

I ordered mine from here: http://www.synlubes.com/ since there were no dealers close to me.