What type of fluid to use in rear axle on 99 3.0 auto trans 2wd

[91stranger]

Title says it all. Need to change the fluid in the rear axle so I gander at my owners manual and sure enough there it is. Motorcraft 80w90 but it doesn't say if it needs the friction modifier or not. Anyone know if this needs it or not? Does it have to be 80w90 or is there another weight that works better? Seems like half the people on here use what they want when it comes to oil... My manual says to use 5w20 but I've only used 5w30. Hasn't hurt anything. Just wanted to see what kind of answers I would get for this. BTW it looks like it has been done before because it has blue RTV around the cover. Should I use blue RTV again or should I opt for the gasket? I've never had issues with rtv but had issues with gaskets...

 

[Shran]

80w90 is fine. Use synthetic fluid if you want, not necessary though. Friction modifier is only used if you've got a limited slip.

I've used both gaskets and RTV. Either is fine. Gaskets are easier if you need the truck on the road in 20 minutes and can't wait hours for the RTV to dry completely.

 

[91stranger]

This is an Ohio truck so it has excessive rust on everything BTW. I'd like to use RTV but I may have to do this on my lunch break so gasket may be the way to go.

 

[Shran]

Yeah I'd use a gasket there too. Make sure the diff cover isn't bent, clean out the housing really well with brake cleaner, clean up the mounting surface, and use some high tack gasket adhesive on both sides of the gasket. RTV is far more forgiving with bolt torque and warped diff covers but it takes quite a while to dry.

 

[91stranger]

Thanks