Well I'm far from a tribologist but I suspect the simple answer would be that most synthetic oils are very broad in their spectrum of operating conditions. Whether that be high and low operating temperatures, their viscosity indices, or their ability to suspend soot and contaminants.
All of this is made even more complex with the latest rounds of engine emission specifications and engineering, and the requirement for diesel particulate filters. The soot characteristics are even more in focus as the wrong oil will kill a DPF very quickly.
Under most circumstances, I would think you would be very hard pressed to find a mineral, or even semi-syn which could match the specification that the manufacturers deem appropriate for their engine.
Since they designed and built the engine I suspect they would have a vested interest in attempting to ensure its longevity, and not simply the profits of the oil companies. Maintenance opportunities aside I don't think most people would be too complimentary of an engine (particularly a modern diesel) that didn't last at least half a million kilometers before wear and tear were noticeable.
Tolerances, and emissions are significantly tighter these days on diesels than those of yesteryear and this has meant that fluids (yes even the fuel) have had to adapt accordingly. Oil has to play its part in regulating the engine temperature, supplementary to the coollant system.
I also suspect it may be possible to achieve the same ends with a non-polymer (ester?), or mineral stock, but the cost might be even more prohibitive than the current range of synthetics to produce.
Them's my thoughts anyway.
Like Gav and Jim said indirectly, where are all our tribologists when you want one?
Cheers,
Iain
Iain
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