Automotive oil pumps (positive displacement gear pumps) are not really designed for a water environment. Oh they'll pump the water fine, but you may end up with rust and corrosion problems.
Or not....hard to say.
But gear pumps are designed with (and must have) very fine tolerances and close fitting gears and end plates. A little rust, a little abrasive action, this could damage the pump.
Alternatively a centrifugal pump does not depend on very fine and close tolerances to work. Here's one from Northern Tool for 79 bucks:
http://www.northerntool.com/shop/tools/product_7738_7738
There are important differences, the gear pump is a positive displacement pump, meaning it will pump water, oil or air, and because of that is self priming.
The centrifugal pump is not self priming, so it needs to stay full of water and/or be below the water source for it to start automatically.
The Gear pump can produce far greater pressures if the water has to go up any meaningful distance, often referred to as the head. It could push it up 40 vertical feet almost as fast as 4 vertical feet.
The centrifugal pump quickly loses pressure if there is much head.
Here is a similar application with a similar Harbor Freight centr. pump.
https://www.youtube.com/watch?v=5_xduog_Kdw
Does that clarify anything?