I will be reviewing some gear oils I've used. All 3 of these are advertised as full synthetic.
I've used either amsoil or redline in all my differentials without any issues and they work well.
Amsoil severe gear 75w90 GL5
9/10 Due to higher pricing. Usually don't need very much and it lasts a long time before needing a change.
Never had a problem with it and it works well.https://www.amsoil.com/shop/by-product/gear-lube/severe-gear-75w-90/
Redline 75w90 GL5 gear oil
9/10 Due to higher pricing. I use this interchangeably with the amsoil and never had a problem with this either.http://www.redlineoil.com/product.aspx?product=57904
Syngard 75w90 GL5 limited slip gear oil
5/10 it was half the price of redline but gave me problems. Maybe other cars are able to use this oil better.
I've recently decided to try something different and picked up some of this.
Right after the fluid change in my rear diff it started being really noisy in every low speed tight turn. I went to a parking lot and did a bunch of slow circles in both directions to get the fluid moved around and the noise went away after over 5 turns each way. Problem is after the car sits for a while the noise just comes back again.
replaced it with some redline and immediately the rear diff problem is gone.http://syngardoil.com/allproducts/syngard-gl-5-limited-slip-gear-oil/