I got that too. In my case it was oil mist from when I got the car undersprayed. Nasty stuff. Squeegee at gas station didn't help at all. Went to a do it yourself wash and turned the foam brush on it. Lift the wiper blades and foam brush them too get get at the oil.

All good now.

98 GL sport (V6 MTX) The Durable Duratec? - (DMD, Metal impeller WaterPump, Synth & Stinky in the tranny)