This is all smoke and mirrors. Change the oil every 5k like Ford says to. Any sooner and you are wasting money.
In 91 I bought a new AWD Aerostar, 12 month, 12k warrenty, and 5k oil changes.
Nowdays its 36/36 warrenty and 5k oil changes, think Ford would recommend 5k if it resulted in increased warrenty claims?
Oil is formulated better, filters are better, engines are built to closer tolerences with better metel. This isnt the 50s anymore, alot has been improved in the last 50 years. When I was a kid it was points, plugs, cap, rotor, and valve adjustment every 12k, with oil changes every 3k. Things have sure changed, now some engines are 100k between tune ups.
Change it at 3k if you must, but your not doing your engine any favors, your just wasting money.
I dont drive my Mustang as much, about 300-500 miles a month, I change the oil at 5k, I just make sure that every time I do drive it I get it up to operating temp for at least 30 minutes.
WM, Jiffy Lube, some dealerships all tout the 3k schedule because most drivers put 15-20k a year on thier cars, thats a difference between doing 7 vs 4 oil changes a year. Multiply that by the thousands of people who use the same business to do thier oil changes. You are taking about alot of money there. It also gets your car into thier shop more often, and usually they try and sell you additional services. I went into Ford to get my first oil change and they wanted to change my oil, rotate tires, and do a coolant system check up for $90. All this for a car with 5k on it. Right.