hi
on diesels with dpf's, its depends how you drive it
if you potter around every where and the revs never go over 1500 rpm your allways going to have problems
give the car a young lad and I can guarantee you will never see a dpf warning ever
its not speed thats required, its revs, if the dpf gets hot enough, the content burn off and exit as soot, keeping the revs up for regular intervals help the dpf heat up and keeping it clear
when the warning light comes on means its failed to clean itself and theres enough back pressure to warn the driver that a manual regen is required to burn off the soot blocking the dpf
if you like to put your foot down regular, then they should be fine, if your going to drive like Miss Daisy, avoid you will get issues with regens
cheers
Joe