Motor oil and chain oil are operating in two totally different environments. Motor oil is in a closed system and is filtered. With this in mind, motor oil is designed to hold contaminants in suspension, thus carrying them away from the moving parts. It is then cleaned by the filter. Viscosity breakdown is the real determining factor on when motor oil should be changed. Unfortunately, it takes laboratory testing to see what is happening, so it in not practical for street cars. Chain oil is in an open environment and is not filtered. So the real factor there is how well it sheds dirt. It will be dirty or will have been washed or thrown off long before viscosity breakdown is a factor.

That is why wax based and synthetic bike specific lubricants work best. They are more self cleaning.