Just an FYI - I did the heads on my sister's 05 Expedition last year. She used to use exclusively ARCO.

While the heads were off, I scraped over 3 tablespoons per cylinder (!) of carbon buildup off of each piston face with a
plastic scraper.

I have done heads on Chevron or Shell powered motors and seen nearly flawless piston faces.
Ask yourself this: At a 20 cents or so difference per gallon, translating to $2.00 per 10 gallons per fill-up, is it really worth the cheapo gas??? Clean engines are more fuel efficient and last longer.
After being read the riot act by me,

she now uses Chevron, Shell, 76, or Texaco exclusively, and is getting better mileage than ever.

Top-tier gas is the only way to go.
Food for thought...
