I would definitely use a synthetic grease like the ones rated for Honda/Toyota cars for example. The bushings are Delrin, so I wouldn't recommend using a standard petroleum based grease with them.
I used synthetic on mine. I have 2 grease guns - one with the synthetic for general purpose and one with wheel bearing grease for the boat and sea doo trailers...:beer:
Prothane Super Grease has very high "tackyness" which won't get squeezed out of the bearings easily and is made for plastic bushings. This is what I use.