/6 water pump???

I never said the quoted statement was true. It surprises me as an engineer because it is so stupid. It also surprised the Robertshaw engineer when he first read it. But, people keep stating it (sorry you agreed with it, learn to judge for yourself, don't follow people who tweet).

As clearly stated - we (i.e. many of us here) think the optimal number of blades depends on the speed that the water pump turns (on average). That varies between options like AC because the pulley diameters vary. Since we don't have access to the engineering design documents, I would try to get a water pump PN listed for your engine and options. If that info isn't available, I would pick an 8-blade over a 6-blade.