Dragboat,
I took a look at the specs in the Castrol 5w-30, synthetic diesel oil ...It appears to be another Group III "synthetic" with a fairly average VI of 158. The Noack volatility is a bit lower than the 5w-40 group III lubes, since it uses a lower level of VI modifier. The MSDS lists it as a "petroleum oil", so it's probably hydroisomerized petroleum oil or a UHVI type basestock.
British Petroleum has a 5w-30 synthetic diesel oil available in Europe - and coming to the US - that looks to be a better product. It's called "Vanellus C8 Ultima" - CI-4/SJ rated - and it's a very high quality, PAO/Ester based formulation that is also suitable for extended drains.
I have no problems running these type oils in gas engines ....I've been using the Series 3000, 5w-30 gas/diesel oil in a number of these applications and it works fine. Sulphated ash is approx 1.5% on this formulation ....