That thing is a time capsule! Simply beautiful piece of history.
Ride it and enjoy it!
As others have said, I'd stay away from group IV and V lubes. And avoid the harsh flush chemicals like seafoam, etc. What you want is slow, controlled, methodical cleaning to occur.
Consider HPL. They make some excellent engine cleaner; many here have had success with it, as it's ester based. Or, even maybe use their "normal" PCEO synthetic; it's a high-purity group III with esters and ANs. And as the BMW does not have an internal wet clutch to worry about, the whole "energy conserving oil" vs. clutch issue is moot.