You are being naive. Otherwise everything about oil Viscosity grades vs. weather connection is extensively explained on these forums.
If you read your cars owners manual with little care, the required engine oil for your car is API CF, CF-4 or ACEA B1, B3, B4, B5 quality with recommended viscosity 5W-30. Stick to these and you are good to go.
Further, generally fully synthetic oils are better than semi-synthetic and semi-synthetics are better than mineral oils.
An xW-40 oil will have generally same viscosity at engine running temperatures. So there is essentially no difference between a 5W-40 or 15W-40 when engine is hot and at running temperature.
But a 5W-40 is always better than 15W-40 since it will give you a lower viscosity when engine is cold and a resultant better cold start and less engine wear during warm up period.
Also, all 15W-40 are mineral based oils while almost all 5W-40 are synthetic oils. So 5W-40 are inherently better oils than any 15W-40.