yes its recommended practice to coat a new impeller with some grease, I personally use a silicone based grease like a dielectic igntion grease but any EP or marine grade grease will do,
Nah sayers will frown at petroleum based grease but at the same time they will use it too, and so will I, its long term exposure to lower unit lube that will cause the rubber to swell and deteriorate so your fine with some common grease gun grease that will wash out after a few hours or less of run time.
Also just my thinking on the melted impeller,three lines of thought here.
1. It ran dry out of the water
2. I see this alot, many times the new impeller fits tight to the shaft and is crammed down to far and cannot float upward with the RPM...make double sure that the drive shaft is clean of crude and that you can slip the new impeller up and down on the shaft semi freely, it must be able to move upward with the pinion as RPM increases.
3. the pump housing is pitted and or has burnt rubber deposits left over causing extreme friction,, If so replace with a new housing
