Fuel economy is probably one aspect of OEM filling with 30w instead of 40w, but not sure how that is a bad thing. A simple tenth of a mile mpg improvement for the average commercial semi truck that averages 120,000 miles a year is almost $1000 in fuel savings. A full mpg better fuel economy rapidly approaches $10K a year in savings. Keep in mind, the move for improved fuel economy is in response to customer demand, and there is no CAFE standards for large commercial trucks. We have one of the clearest examples of the OEM and oil makers meeting the demand of the consumer. A typical semi truck today averages over 25% better fuel economy than just 10 years ago. And there was no government involvement in mandating it!
But engine tolerances are much tighter now. A 30w does an excellent job of protecting the newer motors in these large trucks. Many fleets have fully switched to 30w oils for their trucks and laud the results. I have not seen any instance of degradation of motor longevity when using the 30w oils. Even when going the full 50,000 mile oil change interval that Detroit Diesel recommends now for their DD15 motor. And no, it does not have bypass filtration either. Chevron has done extensive studies on their 10w30 going to 70,000 miles in these applications before starting to approach lower tolerance limits.
But engine tolerances are much tighter now. A 30w does an excellent job of protecting the newer motors in these large trucks. Many fleets have fully switched to 30w oils for their trucks and laud the results. I have not seen any instance of degradation of motor longevity when using the 30w oils. Even when going the full 50,000 mile oil change interval that Detroit Diesel recommends now for their DD15 motor. And no, it does not have bypass filtration either. Chevron has done extensive studies on their 10w30 going to 70,000 miles in these applications before starting to approach lower tolerance limits.