Thin oil = high flow rate = high cooling rate
Thick oil = low flow rate = low cooling rate
Engine failures are often caused by heat build up.
An engine build to tight tolerances needs a thin hot viscosity in order to have sufficient flow to cool. Most modern cars are now 20 weight because they are built so tight. Using thicker oils can cause problems and provide zero benefit.
Typically a thick oil is used on a loose built engine or a worn engine that is beginning to have a lot of blowby. Thick oil can also be used if the temps are elevated sufficiently. However, a good radiator system should keep the engine running reasonably. If you're in a climate that is very hot, it's a better focus to upgrade the cooling system than to up the oil thickness. You can even run a lower temp thermostat and an oil cooler. At the end of the day you should only use a thick oil if the engine is built for or if the engine is worn. Using thick oil for the sake of using thick oil only reduces the cooling performance of the oiling system and can promote failure. These engines aren't built for thick oil and will have no benefit from using thick oil. You step to thick oil when the consumption goes up. If you're burning a quart in under 3k miles, you might consider stepping up to 40 weight. If you're burning a quart in under 3k miles with 40 weight, you can consider stepping to 50 weight. Consumption drops to about half for each step up, at least from my experience. By this point though, you really should be planning a rebuild because the wear's pretty significant.
Also I agree with Scotty. I have no clue how oil viscosity causes CELs. It sounds quite silly. The oil is VERY, VERY thick cold and thins out a HUGE amount as it heats up. If viscosity alone caused CELs, you'd see this happening all the time, especially in colder climates where it takes a significant amount of time for the engine to warm up. It would throw CELs every day you drive the thing.