Originally Posted by JMo

It is possible that [particularly at high speed/revs], the crankcase will pressurize and the oil blow out... if you're lucky, only though the crankcase breather [and into the airbox] as you surmise, but if you're unlucky, past the countershaft output seal, pi55ing oil all over the place...

I'm sure some engine nerd will also elaborate - but I believe it's possible oil could be forced past the valve stem seals too for example?

Anyway, best not to have too much extra oil in there... a 100-200ml might not make much difference [especially on a sump with 2.6litres in], but 300ml might be pushing it a bit?

Of course the XT does keep some of the oil in the frame, so I guess there is a bit more 'breathing space' than on a wet-sump engine, but the principle is the same...

J x