What kind of oil to use in a Wagner paint sprayer?

Many airless paint sprayers do use hydraulic oil. Some use a small amount of hydraulic oil to move a diaphragm pump back and forth, while others use several gallons to supply a hydraulic pump that powers a separate paint pump.

The advantage of using a pump that uses hydraulics is the spray is much smoother, and can be run at a lower pressure than a mechanical machine that allows the pump to run until the top pressure is built, then allows the pressure to drop to a predetermined lower pressure before the pump is started again. This dead band will cause a fluctuation in pressure that is not seen in hydraulic pumps. At higher pressures, this isn't usually a problem; however, it can make the sprayer unusable at lower pressures.