eurostar boiler lockout?

boiler runs for 20 min then burner lock out not a oil problem and boiler is showing proper oil pressure and have replaced jet and phoyocell manufacturer had me drill 5mm hole in fins to get more light to the photocell as runing with brightlight seemed to cure problem but still not going past the 20 min mark. the cumbustion figures are all right as well ,manufacture tec said that they have had lots of calls regarding this lately and advised that i turn the air setting down to grt a brighter flame picture.he said that the fuel oil is not as good as it should be and to add addditive to make it burn better?