engine oil level going up

Hi guys a couple of months ago I noticed the oil on my car was high around 7 liters so I decided to change the oil when I drained it there was no oil and water mixed so I thought maybe whoever serviced it last made as mistake I just checked the oil and it seems to of gone up again after my service I put in around 4.2 liters of oil and a bottle of stp oil treatment what are the likely causes of this I'm guessing at pistons and cylender head and how would I get it checked it has to be fuel in the mix as I said there is no water in there and when I drained it was all oil based getting worried had problems with pressure in the expantion tank (no fluid loss) no smoke no worrying noises from the engine of any kind starts first time fuel consumption is around what I expect about 450 miles to a full tank on my normal business run

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