See where they coming from. but ive seen a nice fair share of dodgy dealer work....and they meant to be manufacturer trained specific mechanics.......hmmmmm....Thats how diesel oils work, they absorb the sooty build ups.
Have had several customers (at a Merc dealers) threaten to take court action with oil samples as the van has just had an oil change yet its black a few days later!
They just won't beleive that a dealership may actually have a clue about what they are doing! To quick to place judgement, the local indies don't always know best!