I have a 1993 merc 200 efi it had over 500 hrs. on it when the rings on no. one cylinder let go, dint know why but it did the other cylinder were fine they had 90 lbs on each one, thew the hole time I never had a problem with the saddle tank. it never got low even if the main tank was low, I think their may be something going on with the main tank.
When my mechanic rebuilt the top end. he Bord no 1 cyl and sleeved it with a new piston and new rings for the other 5, he got a kit to disable the oil injection and I mix the oil 50 to 1, he said if I could see the plastic shaft that runs the oil injection that I would be worried all the time that it would break, so I mix it and forget about the oil thing

Gary