Differential Fluid

My 1969 Mach 1 has a limited slip rear- do I use regular hypoid 85-90w + additive, or do I have to use a special gear oil AND the limited slip additive? What do you recommend for daily driver limited slip rear fluids?

You can use the regular gear oil or syn. But you do have to have the additive for either one. Syn is a better fluid, even with the extra cost.

So there's no special gear oil for limited slip differentials other than the additive itself?

No

The limited slip gear oil is the same but is supposed to have the additives in it

However, I have never seen a case that it has enough for a good clutch type posi. It works fine when they are all worn out, but do the clutches a favor and get some good additive.

My favorite is sold by Kendall in a small bottle, second and equal is Mopar brand, and Ford makes a decent one too but they make two types now, and I havent used the newest style to give you an opinion.

One bottle, then fill with gear oil. You cant over do it, too much only wastes money, but not enough eats clutches

Thanks, I think I'll live large and dump two of the little additive bottles in from Napa and top off with 85w-90. I appreciate your taking the time to answer.