Originally Posted by downwardspiral
The biggest advancement with modern oil has to do with the roller cam. Flat tappet cams need ZDDP (Zinc Dialkyl-Dithio-Phosphate) which essentially fights detergents in oils. Zinc causes the oil to break down quicker and is essentially used in the motor as a lubricant in metal to metal parts like lifters. Roller cams don't need as much zinc, so higher detergent levels can be used. Synthetic molecules have less contaminants because there are no dinosaur bits in it and have higher molecular shear charicteristics than conventional oils, especially when compared over time.
Now here's a guy who knows his oil, as my friend from Mobil did. We change oil on the semi every 10K miles. Remember what I said, use has a lot to do with it. I have a high idle time, sitting while waiting to load, unload, or sleeping....pulling heavy loads, in and out of muddy, or slick conditions. The truck has a heavy duty job, so changes are important. Taken care of properly, there isn't any reason a truck (cummins) can't live for a million miles. Been there done that...the one I own now just turned 400K...