"MUST" use... NO! While it IS true that as an engine wears, the clearances between moving parts get larger, the difference can only be measured with a micrometer. A vehicle would have well over a hundred thousand miles on it before a heavier weight oil would be needed.
No, not anymore. Engine technology has advanced.
The weight of the oil is dependent on how close the tolerances are in production. Newer vehicles have closer tolerances.
Use the weight oil designated on the oil filler cap or in the service manual. Don't go with "thicker" oil. That just messes up the hydraulic lifters.
Changing your oil every 3,000 miles will help you get over 250,000 miles of driving.
Better gas mileage is what's driving the change. Less viscous = less friction. Less friction=better gas mileage.
You should use the oil grade as per the manual. There are special anti smoking additives to put in the oil of worn engines in cars so they do not smoke from the exhaust and attract the police who would put your car off the road. If you damage your car under warranty and they find you had put a wrong grade oil in the engine they may void the warranty if they believe it may have been a factor in the engine malfunctioning. They may send a sample of the oil for analysis and it would be revealed it was not the correct grade of oil and it may be the cause of the engine damage. Specific grades of oil are made to allow the engine to be lubricated as soon as possible for the temperature range in your area. Thicker oil may not pump correctly in a very cold engine risking the vehicle starving of oil and I have actually heard of an engine seizing during a very cold start up through the oil being too thick to pump and lubricate the engine.
Yes, many newer engines have high flow oil pumps that pressurize the VVT servo and timing chain tensioner.
Use the proper grade of oil the manufacturer recommends, not doing so could be a disastrous engine ending.
The millage has nothing to do with it, the engine was designed to use a thinner high flow oil type on purpose.
VVT or fuel on demand controlled lifters bleed off oil fast and employ a high flow melling style oil pump design.
Auto engineers try to run the thinnest oil that does the job. Newer cars generally use lighter oils due to:
Tighter engine tolerances - Loose engines are noisy and dirty engines (oil burn, blow-by etc). Thinner oils can circulate faster and squeeze into these smaller gaps, maximising the motor efficiency and reducing environment killing nasties.
Fuel economy - thinner oils offer less resistance to reciprocating parts. Less power is absorbed just turning the engine. Engines are more efficient.
Superior oil technology - thin oils used to break down under heavy loads and high temperature. Not so much these days. Modern (especially synthetic) oils can maintain film strength under extreme conditions.
Turbos - need a lot of clean, thin oil to lube bearings spinning at 100K rpm. Small turbo engines have the power and economy, but required a jump in oil technology to become mainstream viable.
All foreign and domestic cars nowadays use either 0W-20, 5W-20 OR 5W-30 for the life of the vehicle. The manual in the glove compartment will explain which viscosity oil to use. To the best of my knowledge all automotive and truck companies now recommend full synthetic motor oil.
It's not so much the higher miles as the standards when the engines were made. Newer ones depend on less viscous oil for lower drag, and as a side benefit they work better in oil pressure driven actuators. "Thinner" oil uses closer bearing clearances. In the days of 40W oil there were a lot of engine bearings with clearances up to .005 inch. In the days of 5W-30 oil .001-.002 was common. To use 0W-20, which is becoming the new standard, clearances are even lower.
There is a common belief bearings wear so higher mileage cars should have more viscous oil. The babbit bearings used on the rotating parts don't wear unless they are starved for oil, then they progress rapidly to failure... usually the infamous "rod knock."
Finally, there is the old practice of switching to more viscous oil when the engine starts burning oil. It made some sense; the thicker oil tended to be burned a bit slower, but at the price of better lubrication. In the old days the mantra was "oil pressure" but today it is "oil flow."