oil in the water? no way. Dont worry about that.
There are many many large air supply for fish pumps that feed 4+ air lines.
Remember air pumps only push out around 3 - 4 psi.
Now you can add a oil catch can to a air compressor and no oil or water will be returned to your water supply system. Think about it, they paint cars and such with air compressors and if oil got into paint it would look like crap.
It you think that you need a mass amount of air, I would recommend getting a high volume air
pump for a fish tank not a air compressor. However if you needed to get a air comp. then get a large one, like 25 gallons and set the regulator to 2 psi - just like a fish pump. It wont turn on very much.
I have used the above fish pump to make an air lift system to filter a 6000 gallon fish tank. So I think a compressor is over kill.