I have to question why anyone would buy a diesel and do 5k a year and to then change the oil every 6 months. If you were doing the mileage, I'd agree, but that just seems excessive.
A lot of garages now just buy oil in 5L containers, as there are so many different types/spec recommended by the car makers, they cover their 455 by just getting genuine stuff. It means the customer pays more, but they don't want to ride the risk of future problems. Gone are the days where they keep 10w40 and 5w30 in quantity and it gets one or the other.
There was a 5 year old Jag diesel (the Transit engine one) in my mates garage at weekend. It came in with trouble starting. It had done 15k and really needed a new engine. Apparently, the car had tried literally 100's of times (from ECU read) to run the DPF regen cycle, but due to short journeys, it never got chance to complete. The result? The turbo was screwed and there was a 4mm gap in the piston rings and of course lovely scores and a lip on the bores.
The sales guy who sold that customer a diesel wants shooting.