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Is the owner's manual referring to conventional or synthetic oil?
In the regular conventional oil world, believe it or not 10W30 is better than 10W40.
10W30 is something like 40 percent straight 10 weight oil with the remaining 75 percent being additives to make the oil act like straight 30 weight at operating temp.
10W40 is something like 15 percent straight 10 weight oil with the remaining 90 percent being additive to make the oil act like straight 40 weight at operating temps.
Therefore, 10W30 is composed of more lubricating oil than 10W40. Of course, I have oversimplified this in my explanation, but my point is still holds true.
My current car is turbo-charged, so heat is a major concern. I have put over 200K miles on my engine using Castrol GTX and Valvoline...
straight 30 weight year-round except when climate falls below freezing -- then I run with 10W30.
I am contemplating purchasing a recent model T-bird. Unless I was using a synthetic like Mobile One, I would never use 5W20. If tolerances are really that tight, I would run straight 20 weight. Well maybe, 5W20 below freezing.
5W20??? I would approximate that to be 40 percent straight 5 weight oil with additives to act like straight 20 weight at operating temp. Seems much too thin to me!!!!
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