Hmm, I'm running Mobil1 75w90 synth in my diff. Is it wise to replace it with 75w110?

 

I used Amsoil 75W90 (Series 2000 and Severe Gear, which replaced the Series 2000) for years without a single complaint.
The 75W90 will work perfectly well in the cooler temperatures. Keep it in until summer time - you won't have a problem.

 

I use Amsoil 75/110 or Amsoil 75/140 the Amsoil 75/90 is fine but I like the 75/110 best

 

15K for mountainous areas. I think the regular interval is 30K.

 

I use Amsoil 75/110 or Amsoil 75/140 the Amsoil 75/90 is fine but I like the 75/110 best
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so ill be swapping out my oils. Thanks for clarifying, I thought it was the 75/90 but we have spoken about so many subjects it must have gotten confused(on my part) someplace.

 

Kind of defeats the purpose of buying expensive synthetic for a stock powered street car if you have to drain it every year.

 

You can run 75w90 just fine, the only thing I recommend is that you change it more frequently. Almost every diff fluid appears to shear on this car, so it thins out over time. The longer it runs the thinner it can end up. If you start out with a 75w110 or 75w140 it can run longer before it gets too thin due to shearing. I personally change my diff fluid once a year, well below the recommended mileage. It has to be the easiest and cheapest maintenance you can do on this vehicle. I would never run it close to 30k miles, and I'm far below the 15k interval. it's a personal choice though.

 

It seems that very few people have substantive evidence on when the fluid starts to go bad and needs replacing. Because of this, most folks err on the side of caution like you do. I do find it a bit excessive to change it "far below" 15k miles when Amsoil (who has a vested interest in protecting their customers) recommends a 50k mile change interval under "severe duty" conditions.

I only drive this car about 7-8k miles per year. So long as the change interval is legit, I could go 5 years without changing the fluid and still be ok.

 

I'd have to refer back to some used oil analysis info I have on my diff. fluid sample , but I'd expect a 75w90 to be quite thin after 50k miles. It starts out around a 16.4 cst 100C, I'd hazard a guess it would likely be down to a 13 or possibly 12 viscosity or maybe less ?, and dropping out of the 90 range by then. I am guessing here because I've never run it anywhere near that long. There isn't anything wrong with the Amsoil, every fluid shears down due to the meshing action of the gears, so they all do it over time. It just depends on how long you want to take it. My last run was on Amsoil 75w140, but I'm going back down to a thinner viscosity this year as my runs are so short I don't think it matters. I wasn't overly impressed with the 140 given the short run, so I'm trying out a different viscosity this year.

What I do is definitely overkill, but its maybe a $10 job for me to do the diff once a year. I do the tranny fluid at the same time, my 9 year old tranny shifts like it just drove off the dealer lot, so I'd like to keep it that way as long as possible. Given all of the tranny complaints you read about on these boards I don't have a single shifting issue at any temperature or driving condition, I think that is a testament on an ap1. No diff problems to report either. (knock on wood ). A bit of peace of mind (real or perceived, lol) for a relatively low expense of dumping fluid out a bit early

 

I've been running 75w90 for 5 years with no issues and change about every 15k miles because of track days.