Actually, those Hendrickson grease hub style setups are not technically considered maintenance free. Just because they dont have a "sight glass" on them doesn't mean you shouldn't pull the cap every year or so and do an inspection of the condition of the grease or wheel end adjustment.
Seems to be a huge misconception about oil hub setups being "better" than grease hub setups. You do realize that once the hubs in a grease setup gets to operating temperature, the grease solidifies and essentially lubricates in the same manner as actual oil style setups. You've gain absolutely nothing by switching over.
The bearings, races, the seal, the hub....all the same, just different oil and hub cap assembly. Lack of maintenance was your problem most likely, not the grease.