I'd play it safe and fill the motor's oil reservoir, and use a small premix gas tank. This way you can monitor the oil reservoir to ensure the lube system is working and drawing oil. That 35 year old oil pump has been running dry. Maybe not a good thing for the pump. Or the PO bypassed it for a reason...like it's not working.
Run it like this for only as long as it takes to verify the oil level is dropping. Else you could foul your spark plugs.
BTW. My neighbor bought a used 40 hp Mariner with oil injection. The pump failed his second time out and trashed the motor. Now he has a 25 hp Merc.....with premix.
Seems yours has been running fine with an empty oil reservoir on premix. Top off your onboard gas tank with full gallon amounts and add oil to the correct premix ratio for your motor.
http://www.premixcalculator.com/ .Or use the chart on the oil bottle.