Oil is generally better than grease, if it has seals and oil doesn’t leak out or water get in, I’d use oil. If it doesn’t have seals then John Deere Cornhead grease is the way to go.
I use oil. I have four of them and it seems to work okay. Be advised to look at the prop shaft seal, to make sure it’s all the way in place. They are huge in diameter. I have seen gear case pressure dislodge some of them, due to the force that can be generated on them.