(NSW for work.... language... "WTF? WTF? WTF?...")
GM didn't require synthetic oil for a long time. A friend was the "zone guy" for GM back in the 1990's. He said that they wished everybody would use synthetic in their GM cars, but they didn't require it because the bow tie Bubba's would say "I ain't gonna buy no car that needs $7/quart oil."
Back then, a friend of mine bought a used Oldsmobile for commuting that had regular oil changes, but failed due to sludge shortly after he bought it. He had to buy a new engine, but he got a deal on it from his sister who worked at a Chevy dealer, $1500. I gave him "Autoputzer's synthetic oil speech." He came into work the following Monday saying he looked at Mobil 1 at Walmart over the weekend, and doing an oil change himself would cost $50. I reminded him that his last oil change cost $1500.