In The City's Corporate Yard, where all municipal fleet vehicles go in for service - oil comes in bulk 55 gallon drums. The supplier, who is just a distributor who also supplies bulk oil & filters to any lube shop, wins the bid on pricing. Supplier must supply oil which meets OEM for vehicles in the fleet. It doesn't matter which "brand" is on the drum. So at different deliveries, there are different "brands" on the drum. For all of the police interceptors, light duty pickup trucks, vans, and smaller cars operated by The City, since they are all Ford, it's 5W-20 synthetic blend, which has to meet Ford motor oil spec. The City is not buying boutique brand oil for fleet vehicles. It's not an exact science, with that many cars. Goal is to change the oil as soon as possible, once the vehicle reaches the specified oil change mileage. Not that difficult when oil changes are at 5,000 mile intervals. 5, 10, 15, 20, 25, 30, et cetera. Cars are turned every 3 - 5 years as budget is allocated. However, that still leaves a lot of old vehicles in the fleet because they're still running. If you get 150 new cars this year, you are trying to replace 150 old cars. Makes sense. But last year, 35 new cars were totaled. So you are really only replacing 115 old cars. Fleet manager might also decide to keep some old cars which work, and get rid of a few newer cars with problems. I look out on the street, and still see 90's vintage Crown Vics, a couple of Fox Body Mustangs they drive in parades, the odd Chevy Tahoe and 1 Caprice Classic. Transit division has 3 80's vintage vans with unknown miles.
That's the story in my city.