Possible it could be a leak via the vacuum hose direct into the inlet port caused by a leak inside the carb. but im not over fimillar with this engine so could be totally wrong.
If it is a 1999 model it will be fuel injected, so will not have a mechanical pump. If you are only doing short journeys in the car you can get fuel biuld up in the petrol due to it being on enriched fuel all the time where it is not getting to normal working temperature. The other thing that it could be is if it has a misfire and the unburnt fuel is going into the oil. Mr Vauxhall he has fired a mechanical question at you and you have not answered it.
If it were our historic race engines I'd say you've got carburetion issues, ie the carbs pouring fuel down the cylinders, and for it to reach the sump would mean you've got leaky piston rings. Not sure if that applies to modern engines...but it should be running like a sack of shit, and possibly backfiring.... :lol :If it helps an engine out rebuild on a 1500 precrossflow GT race engine will be at least £1500 thank you very much.
Mr vauxhalls here! hmmm I'm still struggling to see where fuel could get into the oil enough to smell it!!? Never come across this before m, as if it was a misfire, engine would be noticeably under powered, and if it was piston rings trees a good chance te car would be smoking alot, possible cracked head? Coolant mixing with fuel fumes and mixing with oil, hence oil being really runny and smelling of fuel?? Cracked head or headgasket possibly.