Depends on how "Gucci" I want to get, but usually I have a selection of the following on my workbench:
Sticky-G Molybdenum gear grease
Sticky-G silicone o-ring lube
Regular 20 and 70 weight silicone oil
Systema gear grease
Systema o-ring lube
Marine o-ring lube
You don't need a lot of lube, just enough to lightly coat the gears' mating surfaces and a small dab on the piston head and nozzle o-rings will do it.