Ford's answer is to follow the Scheduled Maintenance section of your Owner's Manual, because requirements vary by vehicle. On models with the Intelligent Oil-Life Monitor, the system calculates the interval from your driving, operating conditions and the time since the last service, so you are not tracking mileage yourself.

The instrument cluster gives you two levels of warning. Change Engine Oil Soon means five percent of oil life remains and service should be scheduled. Oil Change Required means the oil life is used up and it needs changing now. Once that message appears, Ford asks you to complete the change within two weeks or 500 miles, and to reset the monitor afterward.

• Follow the Owner's Manual, not a universal mileage rule
• The Oil-Life Monitor reads your actual driving
• Change Engine Oil Soon: 5% oil life left
• Oil Change Required: change it now
• Complete the change within two weeks or 500 miles

There is an outer boundary regardless of what the monitor says. For gas engines Ford instructs owners not to exceed one year or 10,000 miles between service intervals. Diesel owners get a longer mileage limit of 15,000 miles, still capped at one year, and hybrids may travel farther than 10,000 miles between changes but the one-year cap still applies.

If the monitor resets on its own or stops working, Ford's fallback is six months or 5,000 miles from the last change, never exceeding the one-year or 10,000-mile limit. That fallback is also a sensible way to think about the schedule if your Ford predates the monitor.

• Gas engines: never exceed one year or 10,000 miles
• Diesel engines: 15,000-mile limit, still one year
• Hybrids may exceed 10,000 miles; one-year cap holds
• Monitor inoperative: six months or 5,000 miles
• Check your oil level monthly between changes

Ford defines severe driving conditions plainly: short trips under five to ten miles, temperatures well above or below average, dusty conditions, more idling than usual such as heavy traffic, and driving with a heavy load or while towing. Plenty of Peoria driving qualifies, particularly winter short-hop commuting and summer towing.

In Ford's severe-duty schedule most of those conditions still defer to the oil-life monitor rather than a fixed number, with one exception: regular operation in dusty or sandy conditions calls for an oil and filter change every 5,000 miles or six months. One more thing worth knowing is that Ford does not require full synthetic across the board. What it requires is the viscosity grade and the Ford specification listed in your manual, and for the 5W-30 engines that take Ford's WSS-M2C971-A1 spec, Ford names both a synthetic blend and a full synthetic Motorcraft oil that meet it. Ford does prohibit supplemental oil additives, so nothing needs adding to the bottle.

• Short trips, towing, idling and dust all count as severe
• Dusty or sandy operation: every 5,000 miles or six months
• Other severe conditions still follow the monitor
• Ford specifies a viscosity grade and a Ford spec number
• Ford prohibits supplemental oil additives