Oh ok, learn something new everyday, I'd say it would be an oil type of product.
Have they been open or were they removed from sealed systems? If they were sealed but without gas then they should be clean enough, if they ahve been open and are dusty and so on inside give em a blow out. There will be oil that circulates with the refrigerant so any contaminants will be taken through to the dryer/filter. They do handle a fair amount of junk, If you use comp air, Just make sure there's no moisture left in the system as it will freeze and cause blockages in the expansion device(t.x. valve), and it wont work well, if at all.
Or as extra insurance... When you get it charged ask them to break the vacuum with nitrogen (the vacuum will freeze any moisture, then when nitrogen is introduced it will boil off the frozen particles) then vac it again before charging. This will ensure a spotless system, and won't cost much more. Then enjoy the cool