Unless you have just done several fluid changes, try draining as much as you can- one only get's a third of it each time you drop the pan. Best to take the opportunity to replace worn fluid and replace to correct level by as per the precise BMW procedure. This way you knoe it's all gonna be good. If M50/M60, you really need a temp sensor and a long (large diameter) easy syphon tube, to avoid risking the wrong fluid levels.