Thanks Bing! My original premise was that whatever they arrived at would not necessarily apply to the average California driver and that the focus was not on having engines last longer economically, but reducing waste oil.
The city where I live has a varied climate and the city busses come in every 6000 miles for oil and filter changes, and other inspections. All other maintenance is done in multiples of 6000 miles so as to make best use of the time in the shop. The mandate for maintenance is to maximize reliability and availability and also maximize vehicle life at reasonable cost. Used oil goes to a re-refinery.