I just purchased a used 2006 wrx which had about 40k miles on it. By mistake, I filled it up with 89 octane fuel. I now know that I should be using 91 or above. I'm planning on topping off with 93 once I'm at 3/4 of a tank. My questions is this... am I at risk of causing permanent damage to my car after one tank of lower grade fuel? I apologize if this question has been asked before... I'm new here.