I'm familiar with the properties of synthetic oil, just wondering why you are hung up on 10-40. If another, wider rage of suitable oil is available, I'd use that.
Just checked what I have in the basement. I have 0-40 Rotella T SB 100% Synthetic from Canadian Tire. Should be readily available. That was drained and replaced with Amsoil 5-40.
You run into issues when the temperatures run outside your oil's range. There is nothing wrong with running a wider range oil like 0-40 if your engine calls for 10-40.
Here is a good site for you to read: Link
5W30 versus 10W30
Virtually all new passenger vehicles sold in the U.S. use either 5W30 or 10W30 oil. The difference between the two is that the 5W30 flows better when cold, so if you live in a cold climate or operate your vehicle in a cold climate during the winter months, you should use 5W30 if it is the preferred oil for your vehicle. If you live in a sub-tropical climate and don't operate your vehicle in cold climates, then 10W30 is acceptable as long as the manufacturer specifies that it is permissible to use it.
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 www.cartalk.com (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.'
Thicker is Better Myth
The reason that oil viscosities have gotten thinner is because bearing clearances have become smaller. Using thicker oils will interfere with oil flow and the oil pressure will increase. In a worn engine it may be okay to increase the viscosity of the oil because the bearing clearances have become larger.
1991 Mitsubishi Pajero XP TD LWB family hauler.