Wet hubs on boat trailer?

My trailer has the wet hubs that are filled with 90W oil. Is there any maintenance that should be performed on these hubs? If so, how often?

if nothing is leaking,gust check oil level should be good for many miles.if it wories you stop an check hubs for heat they should feel about like hubs on truck.on a hot day if sun is on one side it will feel guite a bit warmer.hope this helps.if seals start to leak you will see oil around them.