the function of a lubrication fluid is to keep dust and dirt away from the piston and avoid chemical reaction of the remaining thin film of component at the piston.
We recommend Mesamoll as a lubrication fluid to avoid further disturbation of your sprayed product.
In case of worn packing rings lubrication fluid might enter the pump between packing ring and high pressure head housing or between packing ring and piston, depending where the packing rings are damaged.
As a result of lubrication fluid entering the pump, in case of Mesamoll your sprayed film is still okay, in case of engine oil you might have to remove some spots cause the oil destroyes adhesion. You might get blisters as well.