To trouble shoot anything, you must isolate various things you can without damaging. If you think it is the riser, take it off or wire it good after warm up or somehow eliminate it temporarily. Do this with any other item you may think of. Your oil test does not prove anything to me. I guess it's good to know you are not chewing up anything. There are many guys on here who could tell you if they could hear it. Most of the mechanics you will find are OBD trained or replace and hope, like me.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn