I change the plugs on my 08 Forester @ 50320miles. The old plugs hadn't changed much with a tight .045-.046 gap. The new were OE FR5AP-11s gapped at .044. The car ran totally different. There is a lot less downshifting (auto) when accelerating and the engine has more torque. I'll change them from now on at 40k miles. I can't tell yet but MPG seems to be increased too.