And yet not all of the engines failed what was the rate of failure before the grade recommendation change and after?
Also passing the buck to the consumer for a design or manufacture failure is rather low.
Maybe Trav will weigh in. IIRC there was no manufacturing defect, the oil spec was wrong for the application, the engineers screwed up. Contrary to popular belief they make mistakes too. All Honda did was issue a TSB for a "thicker oil," and warrantied a boat load of engines that failed. Costly mistake, but obviously they bounced back from it. A count on failures, no clue, would a nfg. ever disclose that? No way! But lets say enough for them to take immediate action, and change the spec on the oil.
When the Ford Modular 4.6L V8 made its debut in the 1991 Lincoln Town Car, Ford spec'd 20W-40 in the Middle East. This was carryover from the old 5.0L OHV V8. The same engine spec'd 5W-30 in the U.S. Motorcraft had a 20W-40 oil available at that time, and that's what many folks around here used. Even though there were "valve seal issues" with the motor, until it was addressed around 1995/1996, it was not an issue in this neck of the woods. The same can be said about the infamous Mitsubishi 6G72 3.0L 12-valve, which spec'd 15W-40 here. I ran an ILSAC spec 10W-30 in mine, because "that's what was spec'd in the U.S." and ended up having valve seal and guide issues 8 years into the vehicle's life. One could argue thicker oil just didn't leak into the combustion chamber due to its higher viscosity, but a lot of times, the valve seals were still pretty pliable when the heads were pulled off for a head job, etc. due to a failed radiator or something else.
When Ford addressed the valve seal issue on the 4.6L, Motorcraft 20W-40 became obsolete and 10W-30 became the default standard in the Middle East - right through to the end of the engine's production cycle in 2012. Vehicles spec'd for the USDM started calling for 5W-20 in MY2001. Ford went as far as increasing oil capacity by 2 quarts for MY2003. I can't comment on whether that was because one of its primary applications was redesigned, or if Ford had issues with engines that had 5 quart oil capacities. Regardless, there are a ton of 4.6L V8s running on oil as thick as 20W-50 around here with zero issues.
Some folks switched to 5W-20, simply because Ford never changed the oil caps with 5W-20 printed on them for export vehicles, and people thought they were doing the engine a "favor" by using what was "spec'd" for the USDM, just like I did with the Mitsubishi 3.0L. Lifter noise was a major complaint in the summer with temperatures over 120°F - you'd be sat in traffic with the AC on full blast and hear the lifters clattering once things really start heating up. Many went back to 10W-30. Some folks, myself included, run 15W-40 HDEO and even 10W-60 with no issues. Friend of mine has been running Liqui-Moly 10W-60 in his 4.6L 2V since he drove his Grand Marquis off the showroom floor in 2005; last I saw the car a couple weeks ago, it had ~482,000 km (301,250 mi) on the original engine and still running strong.
Other manufacturers, especially those selling their own "genuine motor oil", sell 20W-50. Toyota does this, as does Nissan. Mitsubishi and Hyundai/Kia sell their own 15W-40 and Honda sells their own 5W-30 meeting ACEA A3. The Subaru dealer uses and sells 15W-40, Mercedes-Benz, VAG and BMW spec 5W-40. BMW specs 10W-60 for M Series vehicles.