Both drains should be below oil level, BEFORE they tee. If not, pressure will equalize and won't allow for proper draining.
You may have to find or create a port on the sump. Luckily the M62 is provisioned for twin drains
RealOEM.com - Online BMW Parts Catalog
One last thought about oil draining... make the drain as
short and vertical as you can get but absolutely enter the sump below oil level.