Condensation, and possibly a sympton of CCV situ...common, and pages of Threads on this issue, via Search Tab.
Here are some very informative threads. Surprised the OP is surprised, and hadn't read of the possible symptoms and situ.
GL, mD
Yellow 'goo'...see post #9 and on, in this thread:
http://www.xoutpost.com/bmw-sav-foru...iller-cap.html
If a good hot oil change doesn't cure the goo...
http://www.xoutpost.com/bmw-sav-foru...pondering.html
And, a schematic...yes E46, but very similar to E53 situ:
