Your 525i body style is an E39, and your engine code is an M54 (if you are in the USA).
As for the oil drain plug, it is a 17 mm (as I recall) hex head bolt. Don't know how to attach a picture, but if you go to RealOEM.com Online BMW Parts Catalog
and put in your VIN, then follow the links to "Engine" -> "Engine Housing" -> "Oil Pan/Oil Level Indicator" (bottom diagram), you should get the info you need.
Take good care of that E39!