There's an inverted oil system in the PC-9, has the PT6A-62 engine if memory serves me, and can tolerate negative g without adverse effects.
In an upright spin, there will be some positive g rather than negative. If the PC-9 you saw was doing some kind of stall turn / hammerhead type manoueuvre he would probably have had zero or some negative g for a bit, but the oil system can handle negative g including inverted flight for a good length of time, so it wouldn't be a drama.