If you google oil bath hubs and boat trailers you will find an article somewhere detailing why you should change these out to greased hubs. I can't seem to find the article and don't have time to look,
But from what I remember:
1. The big rigs all use these oil bath hubs, however the big rigs don't generally tow some distance and the dunk their axles in the water. Our trailer brakes generate a lot of heat and heats up the oil. When the oil gets hot and the quickly cools it generally takes out either the clear plastic center cap or the rear seal over time.
2. The big rigs are generally trailering in ideal conditions (i.e. the highway), while we take our boat trailers in places where the little plastic center cap can easily get banged up. Once you take out the little plastic cap and the oil begins to leak it only takes about 5-10 miles for your bearings to run dry, get hot, and burn up! You will not make it far without the oil. Conversely with a grease hub system you can bash the heck out of the hub, the bearing buddy, whatever and you still have grease in the system. You can run for hundreds of miles without even the rear seal or the bearing buddy on a grease system.
3. The big rig trailers generally have a good maintenance schedule and have trained mechanics working on them, while most boat trailers have back-yard shade-tree mechanics working on them, usually only when they fail! Parts are cheaper and more plentiful for the grease systems. Bubba's Auto Parts in Page, AZ will have all the parts you need, will Bubba's have the stinking plastic cover for the oil bath system that you need on a Sunday afternoon to get home??
Also, be cautious of China bearings... you get what you pay for! You can't go wrong with a Timken, KYK, F A G, RHP, or Kaydan, but if you see a set of bearings that aren't labeled or have different colored races, bearings, and cages all in the same bearing be advised not to use them. They have different kinds of metal in them and will expand at different temperatures and will end up locking up and failing. I just did my buddy's trailer and he had Chinese cheapos that locked up!
Get your stuff from Kodiak
or from Bearing Buddy