But what I don't get is, the Ford Fiesta owner manual says I should replace the timing belt and ATF at 150,000 miles.
When i asked Ford this, they told me 30 to 50k miles.
Why this discrepancy?
Automakers or the cars they produce want to have "low cost of ownership". Automakers used to say change the oil every 3000 miles, then 5000 miles, now 10,000 miles isn't uncommon. Some automakers say to leave the filter on for (2) intervals. More oil changes, more filters, it costs more to own the vehicle. Years ago, changing transmission fluid involved more labor as well as replacing a filter. I think more fluid is removed and needs replaced this way as well. Or, flushing the fluid which means literally ALL of it (8, 10, 12 quarts or more). Labor plus more fluid, it costs more to own the vehicle.
The dealers don't give a poop about this. More importantly, they get paid more frequently by telling customers to change the oil 2-3x more frequently than the people who developed the vehicle
! You won't hurt the car by doing it more often so dealers are safe in suggesting this. Our son recently bought a '12 Accord and the previous owner had been taking it to an independent shop for service. They put a window sticker on it for when the next oil change was due and combined with the Carfax telling us when the last oil change was, they were "advising" 3000 mile oil changes in 2019 !