Domino, lots of folk who know more than I say it doesn't matter "they will flip the right way when you crank the engine". I've never trusted that theory and always determine which way the pump is turning and insert the impller with the blades positioned in the right direction before replacing the pump cover. I use some sort of lubricant to help the blades withstand a dry start-up. I think it also helps produce a better prime for the pump. I use vaseline, but it is said to deteriorate the impeller blades; so far it doesn't seem to have made a significant difference for me.
Jim
Jim Hemphill
Detour
'87 32' FBS, USCG Certification #1057921
Berthed MM 207 Tennessee River, Picwick State Park Marina