0 (cold "weight")W 30 (hot "weight"), the 0 is only under certain conditions, and when it warms up, 0W30, 5W30, 10W30, and straight 30 weight oil, all have the same properties (all other variables eliminated). I am a mechanic, went to a university for it and everything, most of the car driving public would believe as you do, but you are mistaken.
to quote "how stuff works"
HowStuffWorks "Measuring Motor Oil Viscosity"
"At cold temperatures, the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C, the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot. "
once again, this is fact, not something I came up with out of the blue. now I'm going to really upset everyone and tell them to stop running their tires at max sidewall pressure (or even above manufacturer's recommended), because it actually leads to improper tire wear and handling characteristics, on EVERY CAR. once again, this is FACT, not a guess. on aircraft (and cars), the same tire is used for multiple vehicles with different weights and sidewall loads depending on the width tire and vehicle it's on. how can anyone figure that the same tire pressure can be used on a 4000LB car as it can be on a 2000LB car. the 4000LB car will squish the tire more, making the tire's tread want to go concave, requiring more pressure. a 2000LB car, under the same pressure, will force the tread convex, wearing the middle of the tread every time. if you want to make a difference in this department, run nitrogen in your tires, and run them about 5PSI over recommended (if you want to be most accurate, run them at the pressure air is when the tires are hot, as in right after a drive). nitrogen has the science to back it up. in street cars, it is dimensionally stable to the point where it does not expand (it does on aircraft, but the temperature difference is a hundred degrees plus), and leads to more consistent pressures, and should be reduced temperatures, and thus friction.