Water condensation
 

My 1998 E430 (87,000) has water condensing on the inside of the filler cap. it turns the oil an off yellow color. we drive it mostly on local trips with an average of 10 to 15 miles on both local and interstate roads. so it is warmed up. changed the thermostat last year, no problem there. is this normal? with the car running i can take off the filler cap and the RPM drop about 100 or so. i can place my hand over the opening and the RPM pick back up but i do not detect any vacaum. any ideas