[skyship007]

SAE figures like 5w40 cover a range of possible viscosity figures, so one issue with using a 5w30 rather than an 0w30 is that you might be using a thinner oil at high temps, as most versions of Castrol Edge are near the top of the 30 range.

The other difference is that the viscosity figures quoted for most oils are at 2 temperatures only and viscosity is a curve, so if the figures look the same a 5w30 is in fact slightly thicker when cold or hot than a 0w30. An 0w30 is thinner when cold than an 0w40 in most cases, so using SAE groups is a bit confusing.

The difference between the old single SAE figure straight oils and modern multigrades is achieved by the use of viscosity improver additives, BUT they do slightly weaken the chemical composition of the oil, so in hot desert operations Volvo recommend using a 15w40 full synthetic, both because it is a thicker more high temp shear resistant type of oil and because of ultra fine dust contamination is less of an issue with thicker oils.
The opposite is true in the Artic when you do need the 0w part of an 0w30!

The Germans are very fussy and the chaps I know who work in the local oil lab (ZF) nearly all use 5w40 in the diesels once they are out of warranty. One chap is a bit of an oil fanatic and uses Amsoil 15w40 (One of only a few full synthetic 15w40's made) in summer and LM Synthoil 0w20 plus a can of Ceratec in winter.

Design specs as regards engine oil are still in the low 40's for most non hybrid diesels and mid 30's for most non race petrol engines when new. Most car companies chase fuel economy, so they recommend a grade lower, as oil viscosity does effect fuel economy, both in the engine and in the gearbox. Very few car companies are interested in how long the main block of an engine lasts, as long as it does not fail within the warranty period they will use a cheap thin oil and oil change intervals that are often too long at their dealers.

If you can reduce wear to a minimum it reduces oil consumption in most cases and it's oil burning that ultimately kills a DPF, so oddly enough although a few of my friends are unlucky enough to own diesels with a DPF, they use Acea A3/B4 cat oils in an attempt to keep that wear to a minimum.