I have just bought a 2015 300 factory i dropped the gear oil and found it to be a thick grey gunk that virtually needed poking out, replaced the amount that had come out with gp10 as i have a few bottles left over and the plan was to ride yesterday then drop it again and get the crap out....but during the ride yesterday it is shoving clean oil out what looks like a crankcase vent to the right of the carb below the exhaust silencer? The bike is hunting like crazy and feels very lean especially on downhills my question is could it be the clutch side crank seal and can these be replaced without splitting the cases?

Many thanks 

The grey gunk sounds like water has got in to the box, perhaps from an external source (drowned in water section) or from a water pump leak, there shouldn't be any pressure in the box or clutch case when the engines running, can you run it with the filler plug out and feel anything pulsing? like you say it does sound like a possible crank seal issue.