Benvenuto!
A common mistake is to put modern thinner oil than what the engine was designed for. The result is a low oil pressure reading. These engines were commonly using 15w40 in the winter, and 20w50 in the summer in hotter places like southern Europe. With these oils, with a warm engine, 1kg or 15psi at idle is acceptable. The needle must then go to max reading as soon as you give it some throttle. If you don't have that, you then need to investigate. Your oil pump may not be the problem. Usually worn rod bearings will result in lower oil pressure. Check for metal particles in your oil, at the bottom of the oil pan or in your oil filter which would be an indicator. You need to diagnose your problem precisely as changing parts until the problem is solved can sometimes end up into being a very expensive method, especially given the price of MB parts.
