No, should be fine. Spec allows for Part-Synthetic and a range of viscosities depending on the weather where you live, but 10-40 will be fine and is what I use in the UK's temperate climate.
I would not go for a pure mineral oil though, as the Part-Synthetics are about as low as you should go.
And as has been said in another thread, I would consider it better to change a cheaper oil twice as often, rather than use a more expensive Fully-Synthetic and leave it in there for ages. Manufactures today only quote such long service intervals (which in turn requires such expensive, long-life oil), to make them look good in the "Which Magazine style" running cost comparisons. I changed the cheap, part-synthetic oil in my old Land Rover Discovery every 6000 miles (which was about a 2 month interval at the time) and it was perfect even after 260,000 when we sold it.
