Let me make something clear! GM, Ford and others are recommending lightweight oils because they have to. There is a law called the Moss/Magnesun Act that requires automakers to achieve a certain EPA gas mileage figure for the vehicles they manufacture. It's called Corporate Fuel Economy Averages.
Very large companies like GM play all kinds of games trying to keep their EPA mileage figures below a certain threshold in order to avoid gas guzzler taxes.
One thing they do is recommend very low visosity oil so that they can claim a few tenth's better mileage for the millions of cars they produce. These lightweight oils will not show much of a change in real world situations but they make a difference in the games these companies play with their CAFE averages.
As for GM engineers or Ford engineers or any other American car manufacturer's engineers, their goal isn't to produce world class products, their goal is to CUT COSTS!
Mercedes is pretty clear about what they recommend. Check this URL out:
If you have a Mercedes with the FSS system, look for oil that meets MB specification 229.3. If you have an older car, use 229.3 or one that meets MB specification 229.1.