Yea, think the float or floats are sticking and flooding into the bores. I replaced the seals on the jet housings just before all this carry on started. Those Strombergs are a pain in the arse. Cheers lads.
Ah, just seen your subsequent post: we not the gauge then. Good idea to use some cheap stuff: actually some ASDA stuff I used was quite reasonable for £7.99 in cost and performance. We await to hear what happens. G.
Possibly you've got cooling water into the sump through a leaky head gasket or serious carb flooding leaking petrol past the rings but I don't see how without some other symptoms (poor running etc).
I would refill it with cheap 20/50 oil from Tesco and see what happens. (Don't want to waste a lot of money on expensive oil if it's just going to get ruined in a week!)
If the cheap oil survives a few trips you can change it for some good stuff.
Maybe someone put in some Wynns Engine Flush (or similar) and forget to drain it out - if I remember correctly that stuff comes out like black petrol.