Small self priming water pump clarification

Anyone familiar with whether or not the smaller marine grade self-priming water pumps respond as an on-demand pump? When i speak of smaller, I am making reference to a pump that pumps approximately 3 gallons a minute. I understand the next question may be dependent upon brand/make/model but are these pumps typically made with impellers that can withstand a momentary absence of water? Would 10-15 seconds without an inflow of water, damage the pump? I can only assume that these pumps are designed in such a manner that the impellers will not be damaged in this situation. But, I am not familiar with the smaller type pumps of this capacity?

So, if a self priming water pump is not an on demand pump than what are the distinguishing characteristics?

Also I have heard that a lot of the smaller pumps need to be mounted lower than the water source as they are gravity fed. In my situation I may need the pump to be mounted higher than the water source but have no room for a larger, high end pump to fit in this location. So I am wondering what the pumps termed that are capable of maintaining their prime when elevated above their water source, as in the case with these smaller sized pumps?

Chris

-- Chris Harrell - custom callmaker "Quacky Calls" Eastern NC. http://www.quackycalls.com