I like this version of the discussion a lot better - good points made all around.
As seank said, the only people with the actual data to support or deny the longevity concern are Ford's engineers. All we know is that Ford engines, all of them, will meet Ford's "longevity goals" with the lubricants that Ford specifies in the manual. We don't know what the goals are or whether there is a statistically meaningful difference between various viscosities; we just know that the recommendation "meets or exceeds requirements".
The only solid information we have from Ford regarding their test results is that when the engine is intended to be used in a way that will generate high heat for extended periods of time (Boss 302, GT500, GT Track Package), Ford recommends a specific synthetic 5w-50 product that rapidly shears to the same viscosity as most xw-40 oils. If you intend to use your engine where it will get hot for a long period of time, presumably a thicker oil would be a good idea. But that's a presumption - Ford hasn't said anything about it themselves, nor will they.
As for why Ford wants you to use 5w-50 from Motorcraft, I don't think it has anything to do with needing a 50 weight oil, considering that it really acts like a 40 weight in any case. I think the reason they do it is to make it harder for their dealers to use the wrong oil by accident. We all know that, deserved or not, the reputation of dealership staff is that the only reason they have elbows is to keep their knuckles from dragging on the ground. So Ford came up with a brilliantly simple three-step formula: regular engines get -20, diesel trucks get -40 and sports cars get -50. Hopefully the gorillas can keep three numbers straight. Presumably (there's that word again) if you can count past ten without taking your shoes and socks off, you're welcome to use a shear-resistant xw-40 oil for those hot days at the track.
As seank said, the only people with the actual data to support or deny the longevity concern are Ford's engineers. All we know is that Ford engines, all of them, will meet Ford's "longevity goals" with the lubricants that Ford specifies in the manual. We don't know what the goals are or whether there is a statistically meaningful difference between various viscosities; we just know that the recommendation "meets or exceeds requirements".
The only solid information we have from Ford regarding their test results is that when the engine is intended to be used in a way that will generate high heat for extended periods of time (Boss 302, GT500, GT Track Package), Ford recommends a specific synthetic 5w-50 product that rapidly shears to the same viscosity as most xw-40 oils. If you intend to use your engine where it will get hot for a long period of time, presumably a thicker oil would be a good idea. But that's a presumption - Ford hasn't said anything about it themselves, nor will they.
As for why Ford wants you to use 5w-50 from Motorcraft, I don't think it has anything to do with needing a 50 weight oil, considering that it really acts like a 40 weight in any case. I think the reason they do it is to make it harder for their dealers to use the wrong oil by accident. We all know that, deserved or not, the reputation of dealership staff is that the only reason they have elbows is to keep their knuckles from dragging on the ground. So Ford came up with a brilliantly simple three-step formula: regular engines get -20, diesel trucks get -40 and sports cars get -50. Hopefully the gorillas can keep three numbers straight. Presumably (there's that word again) if you can count past ten without taking your shoes and socks off, you're welcome to use a shear-resistant xw-40 oil for those hot days at the track.
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