What the oil is doing
Engine oil lubricates, cools, cleans and seals. It carries heat away from parts that would otherwise weld themselves together, holds combustion byproducts in suspension so they do not form deposits, and keeps a film between surfaces moving against each other thousands of times a minute.
It degrades doing all of that. Heat breaks the additives down, fuel and moisture dilute it, and the particles it is carrying accumulate. Old oil does every one of those four jobs worse than new oil does.
How often
Follow the interval in the vehicle's own schedule rather than a figure somebody remembers from an older car. Intervals have lengthened considerably with modern oils and engines, and they vary by manufacturer, by engine and by oil specification.
Shorten the interval for severe service, which is more common than most owners think:
- Short trips where the engine rarely reaches full operating temperature.
- Towing or carrying heavy loads.
- Extensive idling, which is normal for work vehicles.
- Dusty conditions, including farm and job site work.
- Extreme heat or cold.
Tell the store how the vehicle is actually used and they can tell you where in the range you belong.
What a change includes
- Draining the old oil and replacing the filter, which matters as much as the oil.
- Refilling with the correct grade and specification for the engine.
- A check around the vehicle while it is up: fluid levels, belts, hoses, tires, lights and anything obviously leaking.
That last part is the quiet value. The oil change is routine. Noticing a perished hose, a worn tire edge or a leak while the vehicle is on the lift is what stops a routine visit turning into a tow six weeks later.
For work vehicles and fleets
Oil changes belong in scheduled maintenance rather than being remembered by whoever is driving. Fleets running fleet services have the interval tracked per unit, which is the difference between maintenance and intention.
Fleet vehicles also tend to run severe service without anyone calling it that: idling, loads, dust and short runs. The interval that suits a commuting sedan rarely suits a service van.