BMW recommends Exxon/Esso Unirex S2 in a service bulletin for clutch replacements. It replaces an older spec grease. They recommend using a paint brush to apply a thin film to the splines.
I used it when I did my clutch but the only place I could find it was at a BMW dealer. If you decide to use it, it's sold as BMW part# "83 23 9 416 138" . It was about $8 for a tube about 1/3 the size of a standard toothpaste tube, so not cheap.