I may be able to help a little bit as karl [NMS] diagnosed the same problem on mine.....

It's a result of possible bore glazing, but new rings and head gasket sorted mine. He did tell me worst case though that the criss crossy bits on the pistons could have worn away/off [how technical am I???? ]which causes oil to get into the fuel or the other way round, christ I'm useless, ask Karl he's a top bloke and will help you out