Most of the new trailers we sell have Dexter "EZ Lube" or other brand axles with grase zerks.  We always advise our customers to avoid using them and to have their bearings hand packed, preferably once a year.  Many people adhere to the theory that if one pump is good, 10 pumps is much better.   The EZ Lube system is reallly job security for us as we do make a living in our shop replacing hubs, axle beams, bearings, brake assemblies, etc. 

When there is not enough grease, obviously the bearings are eventually destroyed, with additional expensive consequences.  When there is too much grease, the pressure can cause the seal to leak.  The result of that is not only loss of grease in the bearings, but we have had trailers come in that had the brake assemblies packed solid with grease.  Usuallly, the trailers were brought in because the brakes didn't seem to be working.  Wonder why.  At that point it is more economical to replace the entire assembly than to pay for the time it takes to clean the grease out and wait for the shoes to crumble.

If you are repacking the bearings every year and you aren't pulling in very wet conditions, or long, high speed, hot trips, any petroleum based wheel bearing grease will do the job.

If you want to extend the repacking schedules and have very wet conditions such as boat trailer immersions, synthetic grease of various brands is the ticket. It won't wash off like the dino grease and won't be contaminated by the water. The bearings will run cooler and longer with the synthetic greases.