I seem to recall another thread that suggested that as a result of a period of negative G, chip detector lights came on for both engines. the gearboxes were found to be rather stuffed as a probable result of lack of lubrication.
I know that the Decathlon has a lubrication system (dry sump from memory) that contains a "slobber pot" and associated hardware to turn the vent into a suction line and vice versa depending on which way the aircraft happens to be oriented, so that there is always some nice pressurised lube flowing. I don't think a Kingair would have this feature (although a PC9 must have)
In other words, if the POH says that the G limit is -0.0 G, it says it for a reason.