Sorry for posting reply here. (I wasn't able to send you PM.)
And Thx alot for your vanos oil leak posting!!!
This is kikimouse (Kyung).
I have an e46 M3 that had vanos cap oil leak. (exactly same as your M5 oil leak.)
I think my car have same part for vanos cap and cap oring as your M5. I just have less since it is inline 6.
My vanos is from Dr vanos (replaced 7 years ago) and it developed slow leak 6 months ago at the cap, and it became really bad last month.
And, thx alot to you, I found your post, and I fixed it.
I ordered the whole set of vanos rebuild kit just to get the two Orings. (No one cells the oring seperate.)
Opened the vanos cap, and found that previous oring was so flat and brittle, when I bent it, it snapped. The oring became like hard plastic in 7 years!
After replacing the oring there is no leak, and car runs perfect.
But I'm thinking... although oil is not leaking anymore, the other orings in the vanos pistons cannot be in good shape.