then I would go for the 15w40, madonna, as you have a little more load on the engine zooming up and down those hills of yours. I wouldn't mess around, Shell's diesel oils are by far the best on the market - they have over 40% share of the diesel vehicle market in the countries in which they operate in - I wonder why......performance.
Gav, what is on measure here is the point at which the lubricant gels out in cold weather and cold start and the second number the upper point at which it vapourises and is no good. It's all about the ability of the oil to hold its viscosity during different engine temperatures.....
As a rule of thumb, 15 W is good down to about -20 degrees C (-25 pumping) 10W to - 25, 0W to -35 (real Arctic conditions). On the upper end, 40 coincidentally is 40C, so that gives plenty of coverage for an engine running hot in the UK summer.....
I'm being a bit simplistic here as to the engineering but hopefully you get the gist.