Yes the
oil does look a little black for only 500 miles on it
As Fixit said...plug it up and see how she flows to the heads
A quick check is to screw in a gauge or a remote line and gage to the sender port ( 1/8 pipe thread) and see what it says.
Use your pre-oiler on the
oil pump as you have.
Once you know whether you have pressure or not, then determine if the gage is hooped or the sender.
Shorting the gage very quickly should make it read full pressure.
Also the resistance of the sender will decrease with pressure.
Tim