Three drops of air tool oil down the spout, once right before using. If I'm going at it for hours I'll do it again.
I got warned off of motor oil by the helpful IR rep who helped me sort out my previous compressor system's performance issues with my then-new IR impact gun. I tore the impact gun down to thoroughly clean it out. It was replacing the restrictive quick release connector with a good one a little later that finally made the biggest difference. But while still on restrictive airflow, I could measure the results with my 1/2" beam torque wrench, and oil type did make a small but measurable difference.
Sure it'll work and won't harm the tool, but motor oil is dragging it down, and proper air tool oil will free up some power. Most guys overcome this by dialing in another 10 or 20 PSI and go about their merry way. I might use motor oil again in a pinch, but proper air tool is cheap and common enough that there's no savings, or reason not to. Heck, even Walmart sells it.