turk182
The viscosity range interpretation you mentioned is true as a basic guide for mineral oils but there are differences. Most synthetic oils can be used as an all season oil, ie, from 5w40/ 10w30/ 5w30/ 5w50 while mineral oils have their limits due to their stability under temp extremes.

To better appreciate the characteristics of an oil, you'll have to refer to the product's spec sheet for kinetic viscosity at 40C/ 100C, viscosoty index, shear strength, pour point, etc. A 15w40 oil has a lower viscosity at 40C but may have ample viscosity at 100C vs a 20w40 oil. A lot of engine wear also happens at start up, thus the importance of the lower viscosity rating since this shows how easily the oil flows when cold- have you ever asked what viscosity nissan used as the initial-fill oil on the patrol? Chances are they used a thinner oil to aid break-in.

On API. I would find out what specific spec of a CF4 oil is required by the engine manufacturer then search the oil data sheets for the same spec in an alternative oil. Most japan diesel engines prefer high TBN to protect from valve stem wear.