If you have a Plymouth, what year was it made? I sat in a 1952 that had a push-button transmission but the used car dealer couldn't start it so I eventually bought a Honda CL77.
Have you already drained this milk? It's transmission oil combined with water created by condensation diluting the oil. It was caused by temperature changes during storage and disuse. If it has stopped draining out, replace the drain plug.
EP90 is one of BMW's recommended fluids. Use it if you've got it. I use 80W90 to accommodate temperature swings in fall. Others here use Redline, Red Devil, Red Menace and a host of other multigrades that deliver—or promise to deliver—improved performance and increased self-worth. You could choose from one of those. Another choice is to refill the transmission, run the moto, drain it, refill it, run it, drain it, refill it, run it, drain it and repeat the process until you believe the transmission contains fluid the quality of mother's milk. No harm in that. Regardless of what course you decide to take, once new fluid has replaced the old, any water remaining in the case will evaporate during use if you actually use the moto instead of ruminating interminably about it like so many here seem to do.
Don't overfill the transmission. The oem rear strut wrench has a mark on its handle that allows it to act as a dipstick to measure oil up to the appropriate level; otherwise, fill with BMW's recommended volume of oil.