Alex, thanks for the info. I still have a question. You stated that "blew a seal" notion is a myth and then stated that over filling : "can cause leakage"... how do you define "blew a seal"? In my book, a blown seal is a leaking seal. We are probably in full agreement but I get stuck on semantics. Scoot On!
A "blown seal" generally refers to a seal that failed (i.e., was blown out) under high pressure, for example in the high-pressure areas of a pump, engine or a pneumatic transmission.
A manual transmission can still have a leaking gasket, but they are generally vented and operate at atmospheric pressure. This kind of failure isn't caused by high pressure.
An overfilled final gearbox on a scooter will discharge oil through its vent hose, which often leads to the air filter box. This doesn't indicate a failed gasket or seal, but rather an overfilled gear box.
Hope this helps.
Alex