6.0L tore down...need advice.

[RUSTY OBS]

yesterday i tore down the 6.0 to see what was causing it not to run when it's hot, I found alot of gunk and a broken screen, But also, On the oil pump, There's a little black ring around the place where it mates the two pipes together i believe
It is the black piece on the pipe coming off the top of the oil pump.

also, where else should i check for possible leaks? i have both valve covers off it at the moment so i'm very open to prodding around before i wrap it up and sling it back together.