[Mart]

Just my opinion, but judging by the condition of the rest of the car I would say the oil pressure is fine. It's just not what you might be used to seeing. As long as some pressure is showing when driving down the road all is well. Very little or even no pressure when idling is common in these older engines.
You say it runs smooth. If no no knocks or bangs then stop worrying about it.
You're not a car guy. I am. If it were me I would change the oil for either straight 30 or 20W50 if it could be said detergent type oil has already been used. I might also be tempted to remove the added on oil filter and plug the feed and drain holes. Trying another mechanical gauge may also be in order.