Proff is not the first person I have known to dislike Duckhams, a plant manager mate who is a seriously good engine technician reckons it cost him a new engine once too, my oilhead currently runs on Morriss of Shrewsburys finest 20w50, its about £6.99 for a gallon, I do a change every 3000, quality filters, not concerned at all. My old R100 used to get the same fare.

I did a course once on thermal analysis equipment, some of the bods on the course were techs for Castrol and Shell, they did spill the beans on which supermarkets own brands were theirs, info too old to impart now though.
Stewart