How long does it take for a cleaning additive to work? Should I use synth oil?

What is your question?

Oil presuure light went on. Shop did a pressure test. Oil pressure on low half of block ok. Passage ways for oil to top of enging clogged. Oil additine was added. Was recommended to chage oil in 2 months. Should i change the oil now or wait to see if this additive will take. Pressure light still comes on but at longer driving intervals than before.