The second number is the only one relevant for a hot engine.
The first or "W" number is only relevant when the engine is cold.
Synthetic Oils are thinner when cold and...much better at maintaining viscosity when hot which makes it possible for example, to have 5W50 weight with synthetics. To have a spread like that with "dino" oil a lot of "pour point depressants" would need to be added to achieve that number.
If you used 5W50 it would be good under any conditions...cold morning starts or extreme heat riding. All pour point depressants do is prevent the oil from getting too thick when cold.
