Nice to know where the return line is, thank you. I never really paid any attention to it.
As far as placing grease, or a thick substance in the pump is not good IMO. A friend tried that and it wouldn't pump oil. It is designed to pump a liquid not a thick compound.
As Jennmarr suggested you might try 10-15 lbs. of air pressure in the tank to pre fill the pump. Remove your ball check valve and watch for oil to come out with the air pressure.
This is really confusing to me. Never had anywhere near this much trouble priming a pump.
tourbox
I just tried this, got oil at the check valve, but I still have no oil pressure.