around 70% of the 5MT gear boxes I have stripped down have failed due to poor lubrication .This mainly afects the side thrust of the gear. The gear is normaly seizes > welded to another mating gear , hence not been able to select another gear. Failure is due to the above oil separating ( layering out ) in the main shaft & no oil being able to be feed to the bearings areas.

The oil holes on the main shaft are around 2mm in od & splash feed off the top of the box via oil tray down the main shaft. Even a small bit of plastic off of a selector fork will block the hole. With the oil separating out ( looking like icing on a cake ) there is no chance for lubrication in the oil galleries, thats were disaster sets in. Dropping the clutch & ripping teeth off is another story.

my self I keep well away from the stuff