2003 f350 with a 5.4... commenced to installing the high octane tune on my truck with the milage around 60000 and was blown away by the performance increase, but because of the rising fuel costs i changed it back to stock at around 90000. I have noticed slight pinging and valve noise since then with 87, if i fill it with 89 it seems to run ok. I would also like to add that I have also recently switched my oil to synthetic ( if that has anything to do with it).
Could anyone explain to me why this is so, or any suggestions?

 

You may need a tune up. New plugs might fix your problem.

 

I had iridium plugs installed at around 60000 miles or so.