I'm reading it as the oil in the gear case... In that case, there are 3 possibilities that come to mind:
1. Someone forgot to put oil in it the last time it was apart
2. Bad gear case seal, or oil pump seal and it leaked out into the belly pan over time
3. Bad crank seal and it was sucked into the crankcase.
If the seals haven't been replaced recently, I'd bet a melted Invader piston that it's #3...