I've got an 06 Cummins and I'm a member of a couple of forums for such... and lots of guys run WMO (filtered of course) through their Diesel Only engines. Had an article a couple of months ago about a guy who runs straight ATF through his Powerstroke. There are a couple of Cummins OTR engines that actually pull WMO from the pan when the onboard computer tells them it's no longer cutting it as motor oil... burns it and replaces it from a tank of clean motor oil. I think those guys are running only a percent or two... but a good diesel that is warmed up will burn just about anything you can shove through the injectors.
So, if your A3 is still under warranty (lol) then I wouldn't recommend it... but otherwise filter it, cut it, run it.
"Too cold to start a fire,
I'm burnin' diesel, burnin' dinosaur bones.
I'll take the river down to Stillwater,
Ride a pack of dogs."
Rusty Cage
Soundgarden and Johnny Cash