Conflicting Requirements Re:Engine Oil Viscosity

OldGuy

Below is some follow-up writing I've found on the subject. Now, I'm more confused than ever:
Is there a disadvantage to using an oil that flows better when cold, i.e. 5W30 versus 10W30?
Sometimes, but usually not. The crux of the issue is this: the bigger the difference between the cold oil viscosity and the hot oil viscosity, the more the volume of viscosity modifiers and the less the volume of base stock. If you are good about following the manufacturer's recommended oil change interval then stick with the 5W30 if that is the preferred oil for your vehicle, even if 10W30 is acceptable in warmer climates. Older cars may specify 10W30 only. This is because they need a little more viscosity when cold to keep a protective film on the cylinder walls. There have been instances where the larger amount of viscosity modifiers that are present in 5W30 have broken down due to excessive heat and have left carbon deposits on the valves, but this is extremely rare. The proper fix would be to reduce the excessive heat, but the workaround was to use an oil with less viscosity modifiers.

Why do many oil change places, even dealerships, use 10W30 instead of 5W30, even when 5W30 is preferred?
According to Car Talk. Car tips, advice, and troubleshooting. (see link in the references section), as well as many mechanics who have posted on usenet, 10W30 is the closest thing to a one size fits all oil. Many older vehicles need 10W30, and most newer vehicles are okay with it in warmer climates. Since many garages don't want to have multiple tanks of bulk oil they choose to carry only 10W30. The advice that Tom & Ray give is correct, 'it would not be a disaster if you used 10W30, but given a choice, go with the manufacturer's recommendation and use the 5W30.'