My 03 325i using Amsoil 5w30 had what others had observed on there oil fill caps. A rusty light brown film covering the inside of my oil cap. I think I remember seeing someone's previous posts with digital photos and I had exactly the same looking rust like appearance. I just starting using LC and FP, it's only been 200 miles since my first add and I checked my oil level this morning and took a look under the oil cap. The rust appearance is almost gone. I don't know what it was, but it looked nasty and I'm happy to see it disappear. My thanks to everyone on the forum. The information is invaluable to me.