Good day! A question has been haunting me. I have a 2007 2 litre Forester XC. Sometimes, I run it on premium fuel, sometimes not. I find that when I run it on regular unleaded, the performance goes to hell after a couple of weeks. It stumbles around 2000 - 3000 rpm, sometimes quite badly. Sometimes it clears up if I fill up with premium. The handbook says 95 is ok. Perhaps the intermittent performance issues are co-incidental but I ask the question anyway. Are flat fours allergic to regular fuel?