Good point. The effect of spiral slipstream alone is small but still significant. What power setting are you using prior to landing? One that allows the slipstream to straighten out, thereby straightening the oil streaks. Also, the oil is in the boundary layer, which has a very low velocity relative to the fuselage surface, and does not necessarily reflect the behavior of the air 1/2" or 1" away from the fuselage. Still, a very good observation, and worth considering.