I use the blue Merc grease.

I just ordered what is called the "Drive Saver". It is specially designed for use with a Stellings extension box. Stellings only has their metal coupler which has no shock absorbing qualities or dampening effects. We're about to install them here shortly. These couplers are made out of a special rubber/nylon material. Cost is about $240 each and they don't require any grease---no zerts.

BTW, are those little grease bosses called "zerks" or "zerts"??? I have always known them as "zerts", but I could be wrong. Anyone have the correct answer on that?