he he..same problem as the OP.
I've been using 5W/30 BMW synthetic or AMSOIL Euro 5W/40 since a got my ti in 2007. No problems with oil leaks until I decided to use Mobil 1 0W/40 = oil seeping from the front of the engine, probably from the oil filter housing gasket or O-ring. I'm also the third owner so the past three probably used dino oil, which also makes the change to synthetic in an older engine more troublesome for leaks (+ more frequent filter changes until the sludge is cleared out)
I'm switching back to a 5W/30 or 40wt. LL-01 or LL-98 Agree that O is too thin for these "old" engines.
FWIW: great oil thread on BMW LL types:
http://www.theoilforums.com/forums/showthread.php?t=3