I have a 1996 auto with locking diff. What oil should I use? I will tow sometimes and drive fast empty almost all other times.

 

What diff? If it is an aftermarket auto locker, no special gear oil is required.....if it is an OEM limited slip unit, regular gear oils can be used but limited slip additive (friction modifier) will be required.

 

I run 75-140 dino oil in mine - 60 mile commute a day at 70-75 and up to 14K towed at times. Synthetics might be better but I probably over maintian mine so the dino works for me. I add the Mopar limited slip additive to mine and the rear is still tight enough that both tires hop on dry pavement when turning and on the throttle. Rear tires wear out twice as fast as the fronts for some reason

 

My friend, who I bought the truck from, said that it has a locker, but I don't know if it's aftermarket. Don't any of the CTDs come with a locker? Would 85W140 be fine?

 

No, there is no factory equipped locker in a Dodge truck of any year - only limited slip diff options.....85-140 is on the heavy side if you operate in cold climates and is really only used when towing really heavy in hot climates.

 

I live near Phoenix in Arizona...
but I don't tow really heavy I guess.
Once in a while the truck will tow a 30 ft pontoon boat - that's kinda heavy especially with stuff loaded on it.

 

Personally, I would use a good synthetic 80-90W for your application.....there is no need to be slinging around 140W unless it is absolutely necessary.