I'm a fan of synthetic oils, but I believe that using synthetic oils for extended periods in diesel engines is not good, because the compression ignition process produces too much soot, sludge and particulates (e.g. carbon) etc. that mix with the oil. Also, diesel (especially in the US) tends to relatively high amounts of sulfur, which will be oxidized and mixed with residual water (if you don't drive long enough to completely heat up the oil and vaporise the water) to produce sulfuric acid (H2SO4), an extremely corrosive chemical for your engine. Therefore, for diesel engines, frequent oil changes are necessary to remove soot/sludge/particulates and sulfuric acid.

I think the only 3 MAJOR reasons to consider using synthetic oils in diesel are:

1) Easier starting in very cold weather.
2) You want to treat your diesel car like a baby.
3) You have plenty of dough to spend.


However, please do not try to use synthetic oil in diesel engines for extended period of time (I think 5000-7500 Miles should be max for synthetic oils). Extended periods (with synthetic oil and 10-15 Kmiles) for gasoline cars where combustion process does not produce much dirty stuff are OK. Modern gasoline engines burn very cleanly.

Eric