If its a gas engine I'm all for synthetic. However a diesel engine oil gets very contaminated very quickly not to mention that there is a lot of it. So I just use the shell rotella and let the engine thing tell me when to change oil. I've got an LBZ, the high schoolers dream engine. It has 198K miles and doesn't seem to use a drop of oil.
OTOH, this is really a religion discussion, after all have you ever heard of anyone having an engine problem due to poor oil quality? Its all good.
All diesels use a little oil, especially at idle. The cylinder pressures are much higher than in a gasoline engine and there is blowby. Diesels have harder rings and when cool along with higher pressures and the harder metal it lets some oil by. When the engine is run harder it heats up and blowby goes down.
A good synthetic in a hot climate like Austin may not be as important but in the northern states when you have such low temps it is very evident on start up. Same thing when you use synthetics in your manual transmission and differentials. A regular lube may take minutes to reach the seals in a rear end but with synthetic its much quicker.
Oil gets dark within a couple hundred miles in diesels but that does not mean it gets the particulate that can damage your engine. With a good synthetic and good filtering including a bypass oil filter to filter out the smaller particles. A standard diesel filter filters down to around 20 microns. A good bypass filter in conjunction with a regular filter will filter down to 2 microns which allow you to continue to use the same oil for a very long time. The 2 micron filter does not filter as fast so you still need the full size filter and the bypass filter needs to be changed every 25 thousand miles. I change mine every 20 plus thousand.
My change schedule is one quart of synthetic oil every 7 thousand miles. I fill the filter with the quart of oil. When I change the bypass 2 micron filter I also fill that with oil. So I use about 4 to 5 quarts of oil every 25 thousand miles. 3 to 4 regular filters and one 2 micron filter.
I have not done an oil analysis in 50 thousand miles. I used to do it every 10 thousand but they all kept coming back the same. Now that I think about it I will do another just out of curiosity. All my previous checks came out like the Cummins commonly gets. The only change was around 85 thousand miles when the copper started rising but checking it was common in my engine. The oil cooler leaches copper around that time. Since then the copper levels went back down. The main reason to add oil when you change the oil filters is that it replaces the oil additives that
My engine is not used as much as before. I used to drive 25k a year. Now its down considerably to around 5 thousand. Total miles is just over 200k on my 2000.
I know, too much info. Sorry.