Premature Oil Aga burner carboning, the picture says it all !!!, the large old school Supply tube can help, not sanctioned by aga , but cheaper on the pocket , less service visits yearly, well it has work for us! Don’t forget fitting a larger tube doesn’t mean more oil being used it as it goes through the control box first that should be calibrated by your engineer every year or at least checked as it only takes 3 minutes per service