Interesting!
Are you suggesting that even fully synthetic oil begins to breakdown before 10K miles or does this last longer, hence the extended period between changes on Petrol engines?
Is there an oil specifically designed for Diesels that allows longer periods between changes given it's cleaning as well as lubricating function, or possibly an additive that has the same effect when used with ordinary oils?
Actually, about 5 yrs ago, there was no oil that would last to 10k - but manufacturers specify 10/12k oil changes, taking into account the extra wear on engine will not be too much of a problem over the 'life' of engine (ie first 60/80k).
Oils have improved slightly, but still do not last to the service interval, esp those ridiculous 20k services. Its all about the manufacturers needing to specify longer service intervals (for fleets), and taking the risk of excessive wear over how long fleet will keep (60k normally).
As the diesels use oil as a cleaner, there is nothing that can be added afaik to extend life.
My car is specified as 10k changes. I do it as soon as oil starts to go dark, around the 3 - 4k mark. Once it starts to go dark, it is either dirty, or breaking down. Either way, time for changing in my book.