I assume you are talking about oil bath front hubs. Oil is used to lube the bearings instead of grease. There should be a line of some sort on the clear plastic sight glass that indicates proper level.It might say something like "fill to here" or "oil level" You can pop out the plug and fill to that level or in some cases, there is a plug on the side. In that case, rotate the plug to the top and fill to the line.
Most use 90 weight gear oil