Re: Grease for impeller?
Not all rubber products like grease. I am not sure what impellers are made out of...probably either Nitrile or Neoprene. If they are Nitrile, the oils and grease are no biggie, as Nitrile is the most common material used in oilfield seal applications. If Neoprene, then it is actually comparably poor resistance to petroleum based products (grease, oil, vaseline). If there is some incompatibility, the rubber "could" swell up to 10%.
As pointed out above, water floods the pump before start up. I do see the value in making it a little slick for installation purposes...but no positive effect for start up. I would go with a very light coat of oil on the inside of the housing so the legs of the impeller will slide in easily, but I would make sure it was Nitrile and not Neoprene.