>>but here in Canada we use 15w40
As per Mr. Smiley's post, a 10W40 semi synthetic would be a more common choice in the UK.
One other difference I would guess at would be that we also tend to leave longer between drain intervals (although there are some British Luddites who follow slightly silly oil changing regimes!)
In my OM606, I use a 5W40 oil that meets 229.5 changed at 9,000 mile intervals - although I'm fairly sure it could stay in for longer.
Starting with a 15 weight oil in the cold conditions of a Canadian winter must put quite a strain on the electrics.