What viscosity and brand gear oil to use?

[NightRiderQ45]

The title sums it up. The differential holds 2.75pts which equals to 1.38 quarts. The manual states that 80w-90 is preferred but you can also use 75w-90. What brand and viscosity are you all using? I want to buy the full synthetic M1 75w-90 but was wondering if any other brands are suitable. Is full synthetic really needed?

I hate that it's 1.38qts! That will be hard to measure and I don't know if overfilling the diff will cause problems. Also read to make sure I'm able to get the fill bolt off before draining right?

[qship96]

Yes,use a full synthetic for best protection and a tiny improvement in mpg and power efficiency- mobil, Redline, Amsoil......they all will do fine. Impossible to overfill as you basically pump it in through fill hole until it dribbles out then replace the plug. I use Redline 80w-90 and change every 25-30K miles- probably could easily double that, but the fluid is only $20 so why not refresh and get rid of wear particles frequently?

[NightRiderQ45]

Thanks for the info. For some reason, I can't find 80w-90 otc for M1, so that's why I want to go with 75w-90. I plan to do this myself while the car is on a lift. By the looks of it, I might be able to put the fluid in with just the needle nose spout.

[qship96]

You may be able to- but it never hurts to go to home depot, etc and purchase a foot or two of vinyl tubing to connect to bottle top just in case! if you end up using a bottle with a wider mouth, then purchase 5-6 feet of tubing and connect a funnel to one end and insert the other end into fill hole and pour away!