2003 passat 1.8

Went to dealer, looking at a 2003 passat 1.8 automatic. Did the usual, checked oil, looked okay didn't smell, not choc brown. Anyways, opened the oil cap, and slight sound of blow by. Could see the oil mixing in there. Car only has 18K miles. Drove on highway and car seemed to pull to the right. And maybe I'm looking for faults, but it seemed to have a side to side sway when I slightly move the steering wheel left/right. When I asked the dealer about the humming sound when oil cap opened, he said that your not suppose to open with engine running? Is this true? I've never heard that you can't open w/engine running? I'm suspious coz it only has 18K and it's a 2003. Should I get a "Carfax" on this car. Anyone with an 03 had this problem? or should I stay away from this car.

suprised the engine didnt stall or set a check light with such a large vacuum loss removing the oil cap.