Not strictly true Dave, whilst you are correct, most filters would become blocked due to debris in the oil the only time I've seen the relief valve in use was when a pattern oil filter was fitted. For some reason it collapsed and looked like someone had grabbed both ends and wrung it out, completely twisted out of shape. The strainer in the oil pan was covered with the paper the filter is made of, only a very small area was left uncovered. It didn't however block the restrictors in the base of the barrels, luckily.
If the relief valve is stuck or sticking you can remove the roll pin in the bolt, you need a 2mm pin punch to do this, this will allow you to remove the spring and the cup and then just clean everything up and reassemble, if you can drop the cup down the bolt and it drops right down to where it rests (which isn't right to the bottom as there is a small shelf it sits on) then it's good to reassemble.
