Some racers back yonder used oil. If you look at Campagnolo hubs from, at least, the early 1980's - you'll note they have a little metal spring-cap over a small hole in their center. If you were riding these in a track-race, your pit-crew would squirt in fresh oil when you came in. If you don't have a magic DeLorean with a time-machine - don't go there. Use grease for long life and better likelihood of trouble-free performance.