Tarik,
Have a look at the handbook that came with the car or a technical manual ( available from coordsport ) There is a chart showing engine oil grade to be used versus ambient air temperature.
From the chart, and the swing of temperatures in Britain, there appears to be several choices of grade that would suit, needless to say it ought to be fully synthetic.
I have gone for 15W-50 Mobil 1 ( I judged 0W-40 mobil one to be too thin ). According to the chart 15W-50 is good for -10 thru |PLS|40 DEG.C. or higher ( check this !)and the Mobil 1 variety is classed for motorsport use.
You can get this from Demon Tweeks and even Halfords now.
I have used this on my 2 previously owned Scooby WRX's and my current EVO 6 with no problems, and I do 600 mile per week, but I do not take my car to the racetrack.
However, Mitsubishi do approve the use of Castrol RS 10W-60 ( see Coordsport on-line parts manual)
http://www.coordsport.co.uk/
Mr Mime