I can’t really answer this question. My daily driver is an '88 Ford Escort with a 1.9L 4 cylinder. The Ford recommended oil when the car was new was 5w30 then a few years later they sent out a memo that it was safe to run 5w20. My brother bought the car new and always ran 10w30 in it until 1993 when he sold it to me with 146K miles on it. I switched it to 10w40 in 1993 and have been running that ever since. The little 1.9 is now at 516K+ miles and never had the head off the engine and is still running great. It finally started using a little bit of oil between changes when it got somewhere between 250-300K miles. It does use/leak a quart now about every 1K miles. I also have another '88 Escort, '94 Escort, '97 Escort, '02 Escort and '96 Contour and run 10w40 in all of them and haven’t had any lubrication problems with any of them. Total miles on the 6 of them combined is about 1,100,000.