It is always easier to push fluid than pull it. The other thing you might check for is any leaks in your delivery piping. If you have even a small leak and are sucking any
air at all you will get cavitation.
I found a sight on the web for heating oil pumps. The first thing it said was that the maximum lift height was 8'. It also gave this formula for figuring your line length and size.
L= Line length in feet
H= Head in feet (amount of lift)
Q= Firing rate in Gallons per hour or GPH
Then for a 3/8" line L=6-.75H
For a 1/2" line L=6-.75H
They also note that any fittings, valves and filters will reduce the maximum line length you can have.
Using these formulas you can see 6-.75(8)=6-6=0 so it does not matter what you GPH burn rate is you won't get it to work.
I looked at:
They talk about booster pumps under technical resources area on the lower left of the page. Read that before trying to rig up anything on your own.