My 2008 Subaru Impreza owners manual recommends a 5w-30 SM oil. The maintenance schedule recommends a 7500 mile OCI for normal or 3750 mile OCI for severe service. Severe service might be defined as short trips, extended idleing, towing, use in high heat, or other heavy duty applications. For Subaru, they recommend, for severe service, API SM in 30, 40, 10w-50, 20W-40, or 20W-50. Probably other manufacturers have similar allowances for different oils. If one were following a severe service OCI, would it not also be logical to use the severe service oils recommended? I'm not tracking why there would not be a correlation between the usage, change interval and oil weight. In other words, if Subaru felt your usage warranted a severe service OCI why they would not openly say you also need to use a heavier weight oil. Or, vice versa, if you follow a 7500 mile OCI then you should only use 5w-30. I got the part about oil changes are good for business, but is there a valid maintenance reason severe service schedules and severe service oils don't go together - at least as described in owners manuals.