My guess is you'll find nothing wrong...
If you think about it - the cooling system is under pressure... and would/should push WATER into the oil - if oil is coming into it (the radiator).... as soon as you'd shut the engine off - the pressure of the radiator would push water into the motor. If you dip the stick and don't have a milkshake.. then my guess is it's just residual oil. Maybe somebody told the owner that the water pump lasts longer when you add a half a quart of oil to the water. Who knows. (I'm being silly).
You don't have lots of white smoke (it would be noticeable) showing evidence of water in the combustion process.... You don't have water in the oil or on the dip stick. The motor runs well.... If the cap is off the radiator and you don't have bubbles puking coolant out of it... If you do a coolant pressure check and it holds..... Then I say -- fire it off and go light up the tires!
