F'ing idiots !!!! (not you... the guy at the parts store)
Yes... you need to remove all the grease, and put in synthetic 75w90 gear lube. The older pump uses small roller bearings, and they need oil to work. The newer pumps have a lot more room inside, and are designed for grease.
With seadoo... the best rule of thumb is... 2-stroke = oil... 4-stroke = grease.
Do me a favor... put your boat in your signature line... or over in your name. (like mine) That way I don't have to keep asking.