Insignia 1.8 petrol AX18XER

Hi new member on here asking for some info from people that know about this insignia 1.8 petrol engine model AX18EXR. I have a provolone that has come up I have the dash oil light coming on after warm up and the revs drop to 750 and the the light comes on at tick over , if revs are raised above 900 and held there it go out . On the snap on reader all is good with the engine on fault codes . But oil feed pressure is up and down till held at 900 revs and then sits level.
The question is what is causing the revs to drop so low new oil and filter change the oil pressure switch as was thinking it could be this at fault . Has anyone come across this before and this engine doesn’t have the pick up seals fitted that cause so much problems in other models, any pointers please . Using right grade of oil and don’t want to use thicker oil grade as this would only mask a problem that is there and need finding out what it is .