Ok, I just bought a basically one owner 90 GT a couple weeks ago. The owner has used SAE 30 weight Valvoline since day one instead 10W-30 as per Ford's recommendation. It's currently got 141,100 miles on the car. I'm wondering if that heavy of an oil would have damaged anything internally after being used for so long? I'm getting ready to switch it back to 10W-30 Valvoline. Thoughts? It's not burning any oil. Just leaking at the back of the valve covers.