I've never felt comfortable with either the drip or bubble methods. Much easier in your case would be to simply remove the 17mm hex plug (just above the white text box in your photo), set the crank to either 14 or 15 degrees ATDC, then use a mirror and flashlight to see if the raised vee [see pg.7] on the internal flywheel is aligned in the center of the plug hole. Incidentally, the lock tool to manually check this position is only ~$30, so any DIY OM603 owner should really have one of these on hand [# 601 589 05 21 00]