I have a 2011 Ranger trail trailer with what the manual says are "oil contained hubs". Owned since new, I am trying to figure out how to change the oil. I'm sure they need it by now. I notice on youtube most trailers have an allen screw to drain and refill the oil. My hubs do not have an allen screw. If I take the hub off to drain, not sure how to put lube back in the hub. I would appreciate any help you can provide. Thanks.