Time to change out the front and rear differential fluids. With respect to the rear differential which is limited slip, has anyone used Valvoline synpower 75-140 which states it is for limited slip differentials? Did you have to add any friction modifier? I'll use 75-90w on the front differential. I've already bought the Valvoline fluids. Thanks.

 

As long as it is the required weight and states it doesn't need friction modifier, you should be fine. There is no need to add additional friction modifier.

The down side to this is that you are not able to "tune" your LS.

 

I use 75/140 in rear 75/90 front. Synthetic. With multiple vehicles I buy it in 5 gallon buckets.
I don’t use the friction modifier unless I feel the rear end doing funny stuff. The lube I use is synthetic and it has modifier added.

I use regular paper gasket. with a thin smear of sealant on both sides. I have the aluminum covers with the drain plug so I don’t really need to open up the diffs.