[twj347]

Are your winter tires new or did you use them last winter also? They could have significantly higher rolling resistance than your summer tires. That, combined with cooler temperatures could explain the drop in fuel efficiency. It's a good idea to do a coastdown test when switching to new tires so you can quantify the difference.

European 30W oils are a little thicker than 30W oils in North America, but you wouldn't notice more than a 1% difference in fuel economy because of this.