Changing the oil but leaving the filter is like pouring clean water into a dirty glass. You save the price of a filter and gamble the engine’s bearings.
What the old filter still holds
An oil filter traps metal wear particles, soot and combustion residue circulating in the oil. By the time a service is due, that filter holds all of it — plus roughly half a litre of old oil. Leave it in place and that is precisely what mixes into your fresh oil in the first seconds after startup.
The real risk: the bypass valve
Every oil filter contains a bypass valve. When the filter clogs — or when cold oil is too thick to pass through it — the valve opens and sends oil to the engine without filtering it. The logic is sound: unfiltered lubrication beats no lubrication.
But run on a saturated filter and the valve stays open permanently. From that point your engine is circulating unfiltered oil, complete with every metal particle in it. Wear accelerates, and you meet the result years later as lost compression and rising oil consumption.
How often
The rule is simple: the filter changes every time the oil does. The interval depends on the oil — commonly 10,000–15,000 km on full synthetics and 7,500–10,000 km on semi-synthetics. Your owner’s manual gives the figure for your engine; if you mostly drive short urban trips with frequent cold starts, work to the lower end.
Three details at fitting
- Oil the gasket. Wipe clean oil onto the new filter’s rubber seal. A dry gasket rolls during fitting and leaks.
- Tighten by hand. About three-quarters of a turn after the gasket meets the mounting face. A wrench crushes the seal.
- Check the old gasket came off. Sometimes it stays stuck to the engine block. Fit a new filter over it and you have a double gasket and immediate loss of oil pressure. This is the most common expensive mistake in a workshop.
What a cheap filter really costs
Filters look identical from the outside. The differences are in the pore structure of the media, its dirt-holding capacity, the quality of the anti-drainback valve and how well the housing takes pressure. A weak anti-drainback valve lets oil fall back into the sump when the engine stops, so the next morning your engine turns for a few seconds without oil. A few seconds, every morning, adds up over years.