Chances are its just condensation dude, if your water level is a-ok then its very unlikely to be your head gasket. If it was head gasket, you get the same sh!t in the cooling system too.
When I sold my wifes polo last summer I had the same shocking discovery!
Basically, before it was replaced with the bimmer it had only ever done 3 or 4 miles a day, then it had sat in the gargage for a while, and been intermittently fired up over the two months it sat there before we put it up for sale.
Half an hour before the first buyer came round I popped the bonnet to check all the fluid levels, lifted all the oil filler cap and it was like a mayonaise factory under there!!!
I ended up cancelling the buyer coming round and had a mechanic check it out. He diagnosed condensation, and he cleaned it all up for me.
The following day the car sold, and I've never had an angry buyer calling or beating down my door, so I guess its still cool!
Clean out mayonaise as best you can and top your oil up. You'll need to change the oil to flush out all the sh!t, and make sure the oil is as hot as poss when you do it - that will lower the viscosity, and hopefully drag out the mayonaise residue with it.
