Just had a look at the manual under trouble shooting. The recomendations are basically worn parts in oil pump, or Oil pressure relif valve not seating. I know it sounds stupid, but did you get out as much oil as you put in ? If there is a discrepancy and no leack, it must be burning it. If the same amount came out, oil pump may well be problem. Mine have run on 20/50 fine for years. I prefer the Halford's Classic 20/50. The manual gives a pump rebuild procedure