Derek,

I think you are absolutely right about using 87 grade octane over 91 possibly being the cause of having to have two valve jobs performed within a 30,000 mile interval. I said exactly the same thing recently when I dropped this car off at my private garage to address the "Check Engine Light". I felt it was a "poor economy"
on my part of trying to save a few dollars every time I filled up this car. I just had an O2 sensor spacer installed for the rear sensor. As soon as the spacer was installed, the CEL immediately went off. The CEL did not have to be reset with a scanner.

At this point of 221,000 miles and 22 years of ownership, I will bite the bullet and begin using 91 grade or higher exclusively. I hope I get another 100,000 miles out of this car with just ordinary maintenance. I hope I have not done untoward damage to the engine and the emissions system based upon of 87 grade. If my recollection serves me correctly, the second valve job was performed at 193,000 miles. I guess I will see. Derek thank you for your advice.

Hugh