The only way to guarantee a proper interval for the way your vehicle is used is by inspection. It it looks dirty or milky, you waited too long, change it. If it feels gritty, you waited too long, change it. If it smells gassy, you waited too long, change it. Rule of thumb intervals can be either wastefull to wallet or bearings.

If intervals are adequate, the inside of the motor will remain as clean as the day it was assembled at the factory, regardless the milage. Varnish/tar buildup is empirical evidence of inadequate interval.