I know a lubrication & oils professional (from within the MG world), and asked his opinion on tne 0/40 and 5/30 issue. You may be interested to hear his views......
" .......the 40 is thicker than the 30 at the same operating temperature, although if the engine is running hotter than it should the 5/40 will thin to a level equivalent to a 5/30 used in a cooler engine. So as to Morgan advising 5/40, well that may be the slightly higher running temps in the engine bay of the Morgan as opposed to the Fords. Modern tendencies are towards lower viscosity oils through the use of tighter clearances and use of synthetic base oils. Modern oils have reduced levels of certain additives to protect the CAT.
To be honest I don't think you can go wrong with Mobil1 which can be bought in 0W40 form as well as 5W30. I would recommend 0W40 as being better simply because the 0W implies it is thinner at cold start than a 5W or 10W but is a 40 grade at operating temp. The 0W will ensure rapid circulation of the oil on start-up. I used it in my KV6 engine for over 100k miles without ill effects, in fact the engine itself was one of the best in terms of fuel economy and no oil usage.
Modern full synthetic formulations have high levels of detergency and with modern electronic engine management, there is no need for diesel oils in modern engines. The Mobil1 range is pricey but then again you don't need to change it very often! I used to get 12k miles easily in the ZT on it. But if your mileage is less than 5k per year then go with a slightly cheaper full synth product.
I think oil pressure is misunderstood as it is oil flow we need to the bearings at the right viscosity, and pressure is only telling us that we have some pressure but not how much flow. Using a thicker oil will slow the flow rate of oil to the bearings yet still show a healthy oil pressure reading! Old engines with clogged arteries will show a high pressure but suffer insufficient flow. Bearings don't like oil too thick any more than they don't like it too thin, and both conditions lead to increased heat generation and increased drag on the engine.
Talking to a few experts from Shell and BP, it would appear Mobil1 is still the one to beat, and they suggest that the traditional oil film on which we rely for the bearing protection is how oils used to be developed in order to ensure good bearing clearance. But, Mobil1 allows for metal-metal contact as the pressures and heat generate a protective layer by squeezing the oil into a thin solid layer with good anti-wear protection to ensure good engine life. This would again prove that this is why we see low pressures at idle with Mobil1".
