I use mineral oil currently. Have used spray gun lube a lot in the past, Vaseline as well. Air tool lube in a pinch. The only places I use it are on the needle where it rides against the housing and on the needle of the trigger.
Devilbiss gun lube. Lube the needle at the packing. I put a thin film on the tapered nozzle seats and the threads of the fluid adjuster.
After assembled one small rinse with reducer and it's ready for next time.