In modern oils, the lower the first number, the winter rating, the better. Most people that have issues with that statement will be referencing shear stability. With older formulations, the lower winter ratings would shear over time/use and that would result in lower overall viscosity and potentially some other issues related to sludge. Although shear stability is a real issue, it becomes less significant with each new generation of viscosity index improvers.
The SAE viscosity grade, the second number, references the viscosity at operating temps. The optimum viscosity for a given application is affected by many things - bearing width and diameter, expected load and RPM, crank flex, temperature peaks, ect. Generally, the BITOG recommendations will suggest that you follow the manufacturer recommendation for this second number. Since engines and expeced use are different, there will be different manufacturer recommendations for this second number.
After you have mastered the kinematic viscosity (KV) issues, the story will change to high temperature high shear viscosity (HTHSV). It is measured at temps above normal engine coolant temps, but it better reflects conditions at lubricated surfaces - bearings, rings, etc. Until the KV story is second nature, I recommend that put the HTHSV on the back burner. However, if you stay here a few months, you will master the HTHSV issues too.