petrol in oil

hi to you all
i bought a 1997 600 monster last year, it had been stood up for a number of years. i change the belts,oil and filters & plugs,i cleaned the carbs and put new petrol in it. i took it for mot today and when i got back i notice the oil level had gone up,i drained about half a litre out to get to were it was before. i only went about four miles to the mot statio ,it must be petrol getting in the oil any ideas how many thanks