So I believe our boat has an Oberdorfer water pump. Visited the local boat stores and couldn't seem to come up with a 1:1 replacement impeller. Don't see any model numbers on the housing, how do you figure this out and order them? I don't have any spares at the moment, which I know is just asking for it. Also, the manual talks about advancing a grease cup on the water pump, but I don't see anything like it? Is that because this is a aftermarket pump?