Oil doesn't harm the nailers. We test for this. When adding oil to the nailer however you end up with oil coming out of the exhaust which is what oil-free tools prevent.
Automatic oilers are fine but you need to watch the oiling rate and potential buildup in the tool as it's at the end of the line. Oil can also rob the system of air volume and flow if it builds up causing reduced power, sluggish cycles, and potential skipping.
Just make sure when you finish with the nailer, you unplug the tool daily to expel oil and to monitor the oiling rate of the system.