Diesel engine oil characteristics
I thought this might be useful it is a bit old but the body of it is still quite valid.
Oil viscosity is important but so are quite a few other characteristics.
The type of detergent in the oil affects general engine cleanliness and piston ring and ring land cleanliness but also engine bearing life. Bottom end bearings (big ends generally due the more variable loading) can be susceptible to sub surface fatigue. Calcium based detergents as well as being much more thermally stable also have a side affect of protecting against sub surface fatigue failures.
Due the the emissions equipment on our cars, primary cat and DPF certain oils cannot be used. those being the ones typically higher in anti-wear additives like ZDDP and to a lesser degree the level of alkalinity.
If for some reason those were not there then we can look solely at the oil from a performance/protection aspect. Now we can look for a high TBN (total base number) to protect from acidity from combustion products, high calcium levels (3000ppm +), high levels of zinc and phophorous to indicate good levels of ZDDP.
Try to use an oil with low volatility to reduce consumption via it evaporating off.