In a 2008 G35x sedan, Is it safe to use Mobil 1 75W-90 Synthetic in place of 80W-90 (which is recommended in manual)? If not, any 80W-90 synthetic recommendations other than Amsoil?

Also, what intervals should differentials be replaced at? How many total quarts for front and rear are needed?

I know these questions may have been previously asked but just wanted some updated opinions. Thanks!

That is Exactly what i put in at about 30,000 miles, i think it only holds about 2 pints, + or - I have NOT done the front yet,, i think it holds less and is more diff. to do.....

theres castrol, valvoline, pennzoil. theres no oem fluid so the dealers use pennzoil

Just out of curiosity, are you still under warranty?

If you don't have an 80W-90, then you should be OK with a 75W-90, just make sure it is a GL-5 spec. The Premium Maintenance interval calls for diff fluid replacement every 30K miles, but the Service 1 and 2 intervals only call for inspections all the way through the 120k miles that the service book covers.

IMHO, unless you like to spend money, you don't really need a synthetic oil in your diffs unless you are either towing or racing. There just really isn't that much demand on it (given that it's a closed system, not exposed to any combustion byproducts) that a regular dino product, changed at 30k intervals, wouldn't be more than adequate (perhaps even overkill) for the life of the car.

Even less reason to do it at 30k imo. That's its too soon unless ur driving super hard. I'd wait till 60k

Just inspect your rear diff after ~30k and i guarantee you that you will be surprised by how dirty it gets search here and there is a thread on it with that said i changed mine at 45k and it needed it. Also 75w90 is fine if you cant find 80w90

I've had two different dealers here in town tell me that unless you are towing (who tows with a G35??), the diff service should be done at 60k mile intervals. I actually wanted to do it at 30k, but they talked me down and said that it wasn't necessary and that I'd be wasting my money.

I will probably still split the difference and do it at 45k miles.

So you thought I would post AMSOIL, you're right! RedLine 80X90 would be my second choice.
Gary

I changed mine at 60k was still clear.

^ wat was ur mileage?

Imo I think the premium service is more marketing n a way for the dealers to make money by maintaining the car more frequently even if its not necessary. First gen never had this premium schedule.

Changed mine around 80k and I have a feeling the previous owner might have changed it as well since there was some torque seal on the drain plug. It stank like no other and was pretty dirty overall. It took no more than 1 hour to swap out fluid and cost something like 20 bucks in fluids and washer. Well worth it and extremely easy DIY to do in your garage.

My cars an 03 n jus hit 60