Oil filter is a good idea to check. I'd expect that if the oil pickup screen was plugged, it would be symptom of a much bigger issue. Only time I ever saw that was back in the day arco was selling graphite motor oil. Most of the graphite fell out Of suspension and turned to sludge in the bottom of the engine, plugged up the pump, and fried the engine with ~ 30k miles on it. Arco wouldn't pay for the repairs and discontinued the oil soon after that. Amazed arco didn't end up with a class action lawsuit on that one!
Have seen gasket material hang up in the pressure relief valve causing low pressure. Good news is if you are able to achieve 20psi and this is the issue the motor probably is fine as oil film (not pressure) provides the film to prevent metal to metal contact.