Castrol 4T MC oil is a good choice. However, 20W50 is pretty much the spec engine oil weight, unless you live in a very cold climate.
The engine oil drain plug utilizes an aluminum crush washer -these used to be copper before that metal became the
love child of futures investors- to keep annoying drips at bay. Sometimes, a new drain washer comes with the new filter.
Oil filters, $3000.00 rubber o-rings, shims, drain plug washers and much more can be sourced from a vendor I've had good luck with, Beemer Boneyard. Or you can try sourcing Airhead bits at the local BMW emporium. Take along a photo of a Type 247 BMW so they'll have a clue.
http://www.beemerboneyard.com/Do you need a one-piece filter or the two-part "bendy"?
You failed to mention the model year of your bike. That's always helpful info when you ask questions. Consider posting that info and where you hail from in your profile. See the
User CP tab.
I do hope this doesn't start another dreaded Oil Thread. :-/
Monte