Caporegime
12k is too much for a diesel.
It isn't, not at all.
The vehicles at work have oil changes every 120,000km (74.5k miles), all diesel but with long-drain oils used - something like Mobil Delvac http://www.mobil.co.uk/UK-English-L...engine-oils_mobil-delvac-xhp-extra-10w40.aspx - the quality of the engine oil is key, use a crap oil and reduced intervals won't help.
I know a few cabbies that use this type of oil to extend their service intervals, one with a 450k mile - from new - Passat which he services every 25k using long drain oil.