Gosh, that was nice of myself to post that on the 306 Forum.
I got that advice from the 306 forum and having it thicker will mean when engine is hot and run high revs it stays thick, it works ok for me.
But yes, I do a lot of short journeys too and its recommended to go with a thinner "cold start" oil. Thats the number at the starts of the oil spec - e.g. a 10w40 would be a "10" when at "cold". The lower the number the easier the oil will circulate when you first start the engine, and obviously oil getting to moving bits quickly is good for the engine. The later number relates to how thick the oil is when at "running temperatures". The higher the number the thicker it is - generally for the UK climate anything higher than a "40" is to be avoided unless you regularly do track days etc as the engine temperature will be running much hotter than normal. As my quote above says I use a 5w40 oil personally. To be honest it doesn't matter an amazing amount in the UK climate as long as you have 10w40 or better, just don't rev the nuts off the engine for the first 5-10 mins of driving.