Many years ago I rode a moped that hated ICI petrol,I found the cause of the lousy running to be a white cotton-like substance in the carburetor which once cleaned out returned the moped to satisfactory running.I only ever got this problem with ICI petrol.I've also had a car that ran like it was using water as fuel whenever I filled up @ a particular BP station(the local paper carried a story a few weeks later about this BP station spending thousands of pounds having the storage tanks cleaned after a delivery driver had put the wrong fuels into the diesel/petrol storage tanks,made me think perhaps he'd done it before)Another filling station had problems with water contamination(now a funeral home on the site)after heavy rain.

The link failed: Crower Six Stroke Engine

Euu. Just think of a that white emusion in the engine oil if it dosn't run hot enought to evaporate it - wont have to top up the oil just have to remove the water.