For the amount of use your car gets the stock specification 75w-90 is fine. Mobil 1 makes a very good synthetic 75w-90.

If you were driving the car more and/or harder, I would step up to the Mobil 1 75w-140. It provides greater wear protection, and costs more of course.

It’s my rear end lube of choice for many different vehicles, both classic and newer

 

For the amount of use your car gets the stock specification 75w-90 is fine. Mobil 1 makes a very good synthetic 75w-90.

If you were driving the car more and/or harder, I would step up to the Mobil 1 75w-140. It provides greater wear protection, and costs more of course.

It’s my rear end lube of choice for many different vehicles, both classic and newer

Thanks! - I also forgot to ask how much is needed. The books I have give differing amounts and I've seen comments about filling it until it just about flows out of the filler plug. I guess this must be right as there is no way to fill it any further (other than perhaps through the breather)

 

Does it have a traction lock…limited slip?
Or open differential?

 

^^^^ if trac-loc you will need friction modifier. motorcraft 4oz one small bottle added. from the 1965 owners manual. 4 1/2 us pints. 3 and 3/4 imperial pints. listing is for axle associated with the 8cyl im assuming thats the 8" lists 5 U.S. and 4 Imp. pints for 289-4v and the 9"

 

Well, yes/no maybe so. Most good quality GL-4 and up gear oils already have a friction modifier additive package, just gotta read the label to be sure.

 

Lack of or too little friction modifier is noticeable on a street driven car.
Especially on turns or if there is a difference in tire pressure (straight line).

Sometimes stuff like this gets overlooked.
Just trying to help catch noise problem before it starts and start thinking “now what ??!!!”