Adding oil to the oil injection tanks

[tradestation479]

I have a 1989 merc xr4 black max. Part of the warning label is torn off the oil tank and I am unsure when I need to fill it. The part I can read says to add after it has been running fo 5 minutes. Right now its half full. Does it need to be topped off at all times when running? How do I add the oil. Is it pressurized? There are 2 tanks. One with an air line and oil line on top of the cap and another tank with just a cap on it. Anyone know? thanks