How long does it take for a cleaning additive to work? Should I use synth oil? on 2006 Kia Sorento

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.

Asked by for the 2006 Kia Sorento