I would play it safe and drop the transmission. If something really came apart it would damage the flywheel, end of the crank and the input side of the gearbox. PITA - yes, but better than having to fix something that wasn't busted. I think going 15w40 will be fine. It'll hold up better when it's warmed up. I did a change to semi-synthetic on my Hung Dog after I went overdue on an oil change (used 20w50 mineral oil initially) and I lost power so I thought. Now the oil has had a chance to circulate around the engine I think the rings weren't sealing from the mineral oil being gummed up and they're finally expanding. I probably won't go back to mineral oil again and hopefully it'll keep the innards of the block cleaner.