I worked for some time in the US doing vibration testing of bearings in an oil refinery. The bearing operated in a multitude of situations, types and sizes. One of the most impressive setups was where the tapered roller bearings ran in an oil bath, lubricated with oil instead of grease. This form of lubrication gave superior performance compared to lubrication with grease. The bearings ran much cooler and lasted so much longer. Maintenance was also much easier being quicker and cleaner. Any problem with the oil or bearing was instantly seen and the problem if any could be quickly made good.
The wheels on my trailer are lubricated at the moment with grease. The hubs will be coming off next year and oil baths fitted. The most important requirement is that the oil seal surface be as perfect as possible. This is easily achieved by using the stainless steel "seal saver" inserts on the axle that come in the kit or sold separately. The insert is only used on the inner bearing and is sealed (glued) onto the axle with a special silicon. It is essential that the "seal saver" inserts are used when running an oil bath for the bearings. The seal surface on the axle itself is the worst surface to ever run a seal on. The sealing area of the axle is only CNC machined and it's not polished to the degree that is needed for any seal to work properly.
Even if you retained the grease for your bearings, it would be to your great advantage to fit the "seal savers" to your axles as the seals will last much longer and do what they are supposed to do.
These oil baths will definitely be fitted to my axles/hubs next year. You can see all the info about them here,
http://www.tiedown.com/pdf/c753.pdfKody

Never be afraid to ask questions here, Prov. 11:14