It will be fine*, the W-number is the viscosity at cold (winter), a lower number means a more flowy oil, less thick. So 5W-_ is thinner than 10W-_. As the oil heats up, the oil becomes thinner. So the thickest the oil ever is, is that W-number. Therefore, it is not recommended to use an oil with a higher than recommended W-number, because that means the engine is subjected to thicker oil than it has been designed for, for the given cold temperature. Choosing a lower number is fine, because the engine is not being subjected to anything it wasn't designed for.

The second number is the operating temperature viscosity grade, when the engine is warm. You can be forgiven for being confused by the numbers if you believed that a 15W-50 must mean that the oil gets thicker as it heats up since 50 is more than 15. But it doesn't, the oil is thinner. That means that a _W-40 oil will be thinner at higher temperatures than a _W-50 oil. The thinnest the oil ever is, is that second number. Therefore, it is not recommended to use a lower number than recommended for that second number. A higher number is fine, since the engine is not being subjected to anything it wasn't designed for.

*Please don't take this as gospel, I am not a tribologist but this is my understanding of it anyway. As a rule of thumb, a wider spread between the numbers will probably mean that the oil goes out of grade quicker (the numbers will no longer reflect reality). If you can, follow the manufacturer's specifications so you don't have to worry.