1996 Toyota Camry Check Engine Light Code PO401

My check engine light came on and I took car to the mechanic. He did a scan and the code PO401 was indicated. The light reset and the mechanic advised me to give the car a good "workout" and that if the light came on again to contact him. Following a 200 km drive at an average speed of 110 km/hr. and an overnight rest, the check engine light is on again. The mechanic also advised me that I wouldn't do damage driving with the light on, unless it is flashing.
My question: If light is flashing, was does that indicate.
2nd question: Can oil/gas additives have caused this to happen. We have added Lucas Upper Cylinder Lubricant Fuel Injector Cleaner recently and wonder if that has anything to do with the current problem.
Appreciate your comments!!