I think I would go with PENTOSIN since its OE. Never heard of RAVENOL.
You might check with The California Transmission Supply Company... they sell ZF fluid at a reasonable price for the ZF auto gear boxes.
http://www.thectsc.com/catalog/6-speed-bmw-61.html
another ZF place is... Eriksson Industries.
http://www.zftranspart.com/
Dack