Long oil change intervals are common in Europe. Oil changes generate waste. From what I read, the large oil capacities are to help support the extended changes. My 4 cyl Saturn with the supercharged LSJ engine which was made in Germany runs 6.75 quarts. My Mercedes ML350 runs 7.75 quarts. And, both spec full synthetic. Sprinter owners can tell you there is a very specific oil specification.
Considering that many older engines ran less than 4 quarts, a 6K interval with synthetic is probably less wear than 3K with the poorer grade oils available back when the 3K got started. Add in all the friction reductions and better sealing of modern engines, and 10K - 12K intervals are perfectly reasonable. Don't know if I'd be comfortable with 18K on a diesel though, unless I was running lots of freeway miles under light load.