The back story for the demise of the 10W-40 grade is not all related to ring sticking. The CAFE regulations back then were putting a lot of financial pressure on the car manufacturers to improve fuel economy, and every little bit was important. Lighter oils were shown to kick in 1-2% of fuel economy improvement, and so it was in the interest of the car manufacturers use 5W-30s in their CAFE testing. The EPA, however, would only allow it if the product was recommended in the owner’s manual, was readily available to consumers, and was sold at a reasonable price. 5W-30 was not.
Oil retailers rated shelf space by profit per square foot, and products that did not move were given little to no space. Back then 10W-40 was considered the ideal oil grade for decades and it was well established as the leading seller, whereas 5W-30s were a suspiciously thin new grade with little demand. In order to meet the EPA requirements, car manufacturers needed to get the 5W-30s positioned on retail shelves, which meant getting 10W-40s off the shelves to make space. Owner’s manual recommendations mostly affected new cars and did not create enough demand for 5W-30s to justify shelf space as quickly as it was needed. Therefore 10W-40s had to go.
GM cleverly used some studies showing ring sticking as being more prevalent among 10W-40s (due to the higher level of polymeric VI improvers) to ban the grade and launch an industry campaign to reduce demand for 10W-40s and open shelf space for 5W-30s. It worked and the EPA began allowing 5W-30s in the CAFE testing, saving the car companies millions of dollars.
I don’t doubt that ring sticking was an issue at some level, but I doubt it was the driver in this campaign.