Sounds like your bike has had a hell of a life
Even if it is using oil, as long as you keep it topped up its no big deal. Check it on a level surface, on the centre stand, and leave it for 10m after a ride when it is warmed up. You have got to check the oil the same way each time - it does go up and down a bit if the ground is not level etc.
Just keep it topped up and it will be fine. Happy 'zumaing