Short trips with a cold engine. The lower viscosity of the 5 compared to the 10, when cold, seems to have a beneficial effect for him. It depends on the ratio of long and short trips.
his trips involved several trips to birmingham before the next service, so you could discount the short trips.
If you took your car into, say a MB/smart dealer for a service they would just put in the oil as recommended in the service schedule.
you are right and wrong at the same time brabs.
Mercedes recommend using certain oil brands, viscosity and/or low SAPS oil for diesel engine.
1 dealer may use 0w30 but another dealer may use 5w40, 1 dealer may use Shell and another dealer may use Castrol, 1 dealer may use the same oil for both petrol and diesel engines and another may use 'low SAPS' oil for diesels.
the service scheduled (service sheets) states to replace the oil and filter but doesn't state which oil to use
it all depends on the word from who owns the dealer and where there account is set up... a Pendragon (Stratstones/Evans Ealshaw) dealer may use Castrol, an Inchcape dealer will* use Shell, a JCT600 dealer may use Mobil 1
*
i work for Inchcape and we have to use Shell even though Jaguar state we should use Castrol, so we have both to keep every one happy