You need them. In the center of the engine is a set of gears that spins the rotary valve. They won't run very long without oil. Also... if you try to remove the tank, and just put oil in the case... you may still eat the gears because of a leak, or oil break-down.
It's best to leave it in the hull, and about half full of oil. If you don't get a leak... the oil level will never drop.