in the "70's i used to use diff oil in my air filter because it was thicker--certainly had a distinctive smell if it sucked a bit through or maybe i was thinking of my stuffed PB Vauxhall come to think as ran it in that till it drank the lot lol. Anyway, put a litre in your gearbox & you'll soon know!! & so will the neighbours. Be very unlikely for the center gasket to leak though unless misplaced on assembly or had case damage etc but gotta be an answer there somewhere.