10W40 is 40W when it's warm.. a thinner 10W when cold.
20W40 is also 40W when it's warm.. 20W in a cold engine.
The higher number is the one that's most important. (40W)
If you live in extreem cold climate the thinner 10Wxx will be better when starting up the vehicle untill it gets warm.
In a milder climate with normal cold winters, the 20Wxx should be fine.
The 10W50 or 20W50 or 30W50 weight oils will be thicker than a xxW40 _even AFTER the engine is warm_
This thicker 50W oil needs to be pumped around an engine or gearbox. It is harder to circulate this thicker oil than a 40W. The vehicle manufacturer will specify xx50W only if it's needed.. best to not use it unless it's specified.
You can use the straight SAE 40W oil if xxW40 multiweight is not available. But during (engine / gearbox) warmup the oil will be thick and not lubricate as well as it will later on.. so don't take off until the engine is warm.. and take it easy for a mile or so to give the gearbox time to warm up.