One of the cars is a Mk6 diesel Golf. The manual specifies low-ash oil that, as a practical matter, is only available at the dealer. The reason is that there is a diesel particulate filter that can fill up with ash, and there is no really good way to clean it out. It is at least $2000 to replace.
The VW spec is less than 0.8% ash content; some readily available oils are a little over 1.0%. Would that make a big difference? Probably not. But my VW dealer only charges me $10.99 for a litre of the correct stuff, so I don't feel too gouged. And if anything happens during the warranty period I will have a paper record that I used dealer-supplied oil.