Differential help

[Vooryjoni]

Hello, I plan on changing my front and rear differential fluids soon as well as my transfer case fluid. I have a 98 4runner V6 4wd. I have heard that some people use 80w-90 and others use 75w-90. Which would be the better choice? I think both come in synthetic options. And lastly, which brand would be best? Amsoil or Mobil 1? From what I have heard, Amsoil seems to be slightly better.

Thank you all!

 

[Brian.]

Attached it the owners manual info with regards to viscosity. Deviate only if you know Why (not just because some schmuck posted it on the internet).

As for organic vs synthetic, and brands... it's just opinion. If you think about the situation as a 0 to 100 scale, where 0 is leaving the cases bone dry and 100 is everything absolutely perfect with the most ideal fluid ever.... just having clean fluid at the proper fill level will get you to an 80/100. Having that clean fluid at the proper level and also match the required specs will get you to a 95/100. The last 5 are to be gained by brand, composition, additives, etc...

Just buy what's available locally that matches the specs, and change it regularly. That'll do more for you than obsessing over "best". Me, I tend towards Mobile 1 synthetics, they've done well by me for a few decades in every application I've used them and are available locally no matter where I am.

 

[TheDurk]

Attached it the owners manual info with regards to viscosity. Deviate only if you know Why (not just because some schmuck posted it on the internet).

As for organic vs synthetic, and brands... it's just opinion. If you think about the situation as a 0 to 100 scale, where 0 is leaving the cases bone dry and 100 is everything absolutely perfect with the most ideal fluid ever.... just having clean fluid at the proper fill level will get you to an 80/100. Having that clean fluid at the proper level and also match the required specs will get you to a 95/100. The last 5 are to be gained by brand, composition, additives, etc...

Just buy what's available locally that matches the specs, and change it regularly. That'll do more for you than obsessing over "best". Me, I tend towards Mobile 1 synthetics, they've done well by me for a few decades in every application I've used them and are available locally no matter where I am.

I am right with you except that....

Thing is, I could never find 80w-90 or straight SAE 90 in synthetic so since day one in 1999 (or whenever my first gear oil service was) I have used synthetic 75w-90 everywhere. Proof of this is that Mobile One only makes 75w-90 and 75w-140 in synthetic. In conventional, they only make 80w-90 and 85w-140. Red Line only makes 75w-90 and 80w-140 in GL-5 synthetic. I will guarantee you the dealers don't have two different gear oils, either. (Someone confirmed this a while back.)

Mobil™ gear lube products | Mobil™ Motor Oils

Red Line Synthetic Oil. Gear Oil for Differentials

I can't "prove" it, but my belief is the 75w-90 synthetic is at least as good as the 80w-90 dino in any application or temp. Anyway, 200k and going strong with changes at every 50k. It's a truck diff gearbox, for Pete's sake.

As to brands, I use Red Line MT-90 GL-4 in my 5-speed transmission that I have to order on line (it really is better), but TC and diffs get the Valvoline Synthetic 75w-90 GL-5 that I can pick up at the local NAPA.