[freakray]

Steve,
If the tank has a drain plug at the bottom (it should), when you're certain there is no air in the tank, remove the plug and check what buildup there is in the tank.

On a tank compressor, as long as you're using the air in the tank, and also using a moisture trap, I very much doubt you'll get oil in the air on the airbrush side of the trap.

I would recommend servicing the compressor (including draining and inspecting the tank and cleaning the motor and compressor assembly), and then allowing the tank to go to full pressure (motor shutoff) and then letting the air out through the moisture trap. You could also look for an inline air filter to put in the line - you only need monitor the condition of the element in one of these to know if you've got oil in the air.