What is a good fluid to run in the ZF6 speed. I have used the regular mercon and have some hard shifting issues once in awhile.

 

If your asking about your 2002 F-250 tranny & it calls out auto-tranny fluid, you should be using Mercon-V as this TSB indicates. Fulltext TSBs | Ford Trucks

Mercon-V is a semi-synthetic friction modified fluid that's back specified to replace Mercon. It has a better quality base oil & additive package than the old Mercon recipe, so you might find you'll get better shift quality using it. Let us know how it goes.

 

Will do. I apprecaite the input.