5W40 is a 40 weight just like 15W40. 15W40 offers no better protection in hot weather than does a quality 5W40.
Yes, due to the added VII for the higher viscosity range (5 to 40 vs 15 to 40), it will be somewhat more prone to shearing. In a quality oil over a 5k mile, or even 7.5k mile, OCI...