Just talked with the owner of a disabled passenger transportation company. He uses Ford vans rigged up with wheel chair lifts, seats and other equipment needed for the job. These vans are spec'd for 5w-20 oil but on the first oil change he switches them to Mobil 1 15w-50 oil and Mobil 1 oil filters and changes the oil based on experience with UOA's and past maintenance experience, usually at 10-12K miles with the filter done every other change. His vans usually see between 60 to 75K miles a year driven 2 shifts a day, 7 days a week. He runs them until maintenance expenses start to rise, usually at 400 to 600K miles. He does his own maintenance for most stuff and maintains a yard with crashed or worn out vans for parts and does maintenance for a couple of other operations that use these same vans in their business. He previously used 5w-20 and it worked well for the first 100K miles or so, but after that wear increased to the point where he stated looking for a solution. This increased wear problem was not experienced when he switched to the higher viscosity oil of the same brand of Mobil 1 oil right from the start. I know this is not typical passenger car wear and tear but I though it was at least interesting. Because these vehicles are driven by drivers often times in a bit of a hurry to get to calls with engines left idling to keep the a/c or heater going he said he could not pin down any fuel mileage differences. He also stated that he's never used "high mileage" oil. He knows the theory but has never felt the need to try it. He said his next move to improve maintenance is going to be trying changing the oil based on the amount of fuel used instead of mileage. He thinks that idle time and terrible mileage in rush hour traffic should be considered when it comes time to change the oil. Since all gasoline is purchased from dedicated credit cards and often from the gas station down the street this will be an easy scheme to implement.