Amsoil 80W-140 for rear diff?

[YLW DRM2]

Hey guys, I read a lot of posts in regards of tranny and diff fluid. It seems that unless I want to go the the dealer, which is some 40 miles away from me (all near by dealerships closed) and pay premium on stock fluids, the best bet for the diff is Amsoil. I went to the local speedshop, but they only carried Amsoil 80W-140 in the shop. The guy told me that 80/140 is the same as 75/140. Is that true? What is the difference? Can I really use 80/140 or should I make a special order?

EDITED: Sorry guys, I was typing it late last night and got it wrong about the local shop. The local speed shop carries RedLine 80W140 not Amsoil.

So what do you think? Should I go for a RedLine 80/140?

Thanks.

 

[hightest]

I prefer the amsoil severe gear 75/140.
But the redline isn't bad either.

 

[Tom F&L GoR]

75W-xx means it will be not thicker than 150,000 cP at -40C.
80W-xx means it will be not thicker than 150,000 cP at -26C.

The 75W will allow better fuel economy and less fluid drag when cold. Don't confuse "cold" with what your heater blows on your feet. Driveline fluids warm up very slowly; on the order of 45-60 minutes before reaching steady state.

Most driveline fluids are tested by the OEMs as if they were fill for life, even though drivers are supposed to change them perhaps every 20,000 miles. You have to assume the OEM and factory fill fluids are very good. Another way to word your question is whether the aftermarket fluids have the same or better performance, and how do you get this information and gain the confidence that it does meet or exceed OEM fluids.

 

[YLW DRM2]

I prefer the amsoil severe gear 75/140.
But the redline isn't bad either.

Do I need to add LSD additive with Amsoil gear oil?
Thanks.

 

[hightest]

Both the Amsoil (severe gear only) and the Redline don't require the addition of the slip lock additive due to their synthetic formulations, but if your diff starts to make any clicking noises during turning then the addition of some slip lock additive would be required.
I have not needed to add anything extra to any of my work or sports vehicles that I currently have the severe gear fluid in. All of my vehicles have limited slip differentials.