75w-90 Will make no difference honestly.

 

You can order the good stuff in 75W85 on Amazon. Ravenol, Redline, etc. I'll be changing my rear diff again this summer and have decided to go with the Valvoline flex pack in 75W90 which is fine. You'll likely find that over the years most of the members posting on this forum have used 75W90 when changing their diff fluid.

 

I'm convinced it's so you'll go to the dealer for service since no auto parts stores typically carry 75w-85 gear oil and most people are afraid to use something else. It's the same with trying to change your own brake pads. The front caliper uses an 11mm hex and the rear a 7mm, almost all hex key sets don't give you those sizes...

 

I'll be changing out mine soon and will be using Amsoil 75w85. https://www.amsoil.com/p/amsoil-sev...soil-severe-gear-75w-85-100-synthetic-gear-lube-svl/?code=SVLPK-EA#pills-home_2

 

Transmission engineer here - you’re fine with 75w-90 synth

 

When I did my 2013 wk2 with 128k , used amsoil 75w-90 and had no issues

 

Just be sure to add the Limited Slip additive also.

 

Uh did I miss something or did you not realize that putting a limited slip additive into gear oils in essence makes them a synthetic gear oil instead of non synthetic. Synthetic gear oils 75w-85, 75w-90 which can only be full synthetic to have those designations do to their physical properties and such. So yeah the two mentioned above don’t take or need the added stuff

 

The additive does not change a non-synthetic to synthetic. What it does is change the friction properties so that the clutches in an clutch based limited slip differential do not chatter. Using it won't hurt if you don't need it but not using it if you need it can cause clutch issues.

 

2 year old thread......👆

 

The nice thing about 75W90 is that you can buy the Valvoline in that viscosity right off the shelf at Walmart. It’s full synthetic, has friction modifier already in it (if you need it), it’s a great product, and it’s inexpensive. Best of all it comes in pouches instead of bottles and the pouches are WAY easier to use in tight places such as around differentials.