Personally, I wouldn't use the 10W40. (If I were closer, I'd buy it off you for my other car, mind)! The first number, as m500 said, is the viscosity when cold. 10 is thicker than 5. With an engine like the Exy that seems to be "marginal" in a variety of areas - particularly turbo bearings, I think you could do it a lot of damage using an oil with a thicker cold viscosity. It's true that in absolute terms, the oil will be thinner in summer (if we ever GET a summer, that is!) but I think that's probably in the realms of "might get away with it" rather than "wouldn't do any harm".