A unique website dedicated to fishing information from Florida's Northern Big Bend. This includes the area from the Econfina River west to the Apalachicola River
I haven't done it on my trailer but that is the style tractor trailers use and they go forever. They service them when they put brakes on them. I am sure the marine version would be more water proof and probably more expensive.
I replaced the old hubs on my trailer with a set of oil hubs. Unfortunately I have only towed the boat across town a few times. I am building a boat and have not launched it yet. I should have the trailer in the water in a month or so. So I can't really speak for how the hubs hold up after dunking them in the water. I did tow the boat about 20 miles or so and they worked fine then. The one thing I will say is the surface on your axle has to be in good shape. It can't have rust or be pitted. If the axle surface isn't perfectly smoothe the oil will run out the back under the rear seal. And also with a grease hub if the seal goes bad you might be able to fill it up an limp home. If the seal goes bad or cracks with a oil hub it will all leak out. So you have to check them periodically. But Id rather pop a new seal in periodically than pack bearings. I don't think temperature or distance would matter as long as they have oil in them. Transmissions, engines and rear ends all get very, very hot and the oil keeps them cool. I like the fact that the front covers are clear so they are easy to monitor. By the way my axle wasn't smoothe enough, it was pitted a little so I had to buy some stainless axle sleeves. They almost cost as much as a new axle. So that might add to your cost if the axle is worn. As Chalk said they do have new hub fittings which use air pressure to keep the grease in and water out. They seem to be a good product. I saw a Ship Shape TV episode where they installed them on a tandem axle but they were a little pricey for me at the time. Hope this helps.
I must confess and update my lase response. I was in West Marine the other day and saw an open and returned set of oil bath hubs. The ones I bought were also opened and returned ( I know stupid me ) I figured somebody bought them and the wheel didn't fit over them as they look a little bigger than regular hubs. Anyways after seeing a second pair returned in there I'm beggining to wonder about mine. I see the inside of the hub where the clear cap screws in measures 2.3" so I am considering replacing the cap with buddy bearings of the same size. Mine still leak out the rear seal due to my axle not being perfectly smooth. The stainless axle sleeves are about $25 a piece at Carquest and I can get buddy bearings at $15 for the set. So I will probably go with the buddy bearings and good ol fashioned grease.
Oh yea, it also makes ya wonder about West Marine in less than a year they've had two sets of the things returned and they just tape them up and try to re-sell them. I know the manager seems to be a really nice guy,but whenever I'm in there he's always talking to the other employees about how much $$$$ merchandise they're moving. I guess that's the difference between a nationwide chain and a local business.