Hello,
Yes, you are lucky to still have one of those early water pumps with the oil fill plug. Normally these are long gone and the replacements are sealed. Like so many throw away things in life the new pumps are maintenance free, in other words throw them away when the original lubrication is gone.
Anyway to service the pump remove both the plug on top and the one on the side. carefully fill the pump with 90W until it runs out the side plug hole. Draining the old oil is not required. You will normally find room to add fresh new lube. I remember this as being a standard maintenance item on my 190SL. It was a little work, but I never remember having to change a water pump.
