If you ever use your setup for an air operated tool and use the oiler, you should not use the same air hose for painting as you have contaminated the hose with oil. I would remove the oiler and only use the filt/reg and a clean hose for painting purposes. If you have used the oiler, just emptying it is not enough. Oil will probably still pass, contaminating the clean hose as well as the painting. It's probably a lot easier just to pull the oiler off completely. It looks like a modular unit, and the filt/reg part should be threaded both sides. If oil mist gets in the paint it will cause fish eyes. If you only occasionally use air tools, it's probably easier just to add a couple of drops of air tool oil into the inlet of the tool before each use.