gravity drained. If oil is allowed to (by turning the engine upside down) make it's way against the air seal of the compressor wheel or turbine wheel, it'll seep past the gap and show up in your compressor housing (or exhaust housing).
That should put the matter to rest. That does make me wonder how one typically gets an oil leak aside from a drain blockage. I believe he also said that a bad bearing and shaft play would make the problem worse.