Why it is changed together with the oil
The filter traps the metal particles, soot and deposits that the oil carries. Its capacity is limited: as it loads up, the resistance to flow rises. Once the pressure difference exceeds a preset value, the bypass valve inside the filter opens and oil goes around the filter element — the engine still gets oil, but unfiltered oil.
That is why an old filter on fresh oil is worse than useless: it can return to circulation exactly the particles it had collected. A filter costs a fraction of the oil, and replacing it adds no labour, since the sump is drained anyway.
At what mileage to change it
The usual interval for a modern passenger car is 10,000–15,000 km or 12 months. The one-year term matters even if the car covers little distance: oil oxidises and accumulates condensation regardless of mileage, and the filter ages with it.
Under severe conditions — short urban trips, dusty roads, towing, an LPG installation or repeated cold starts — shorten the interval to 7,000–8,000 km. Engines running on LPG work hotter and load the oil harder than petrol ones.
Signs the filter or the oil is no longer coping
A clogged filter rarely produces direct symptoms, precisely because the bypass valve routes around it. The signals show up indirectly: the oil pressure lamp staying on a second longer at start-up, a short metallic knock from the top of a cold engine, or oil that darkens unusually fast after a change.
If the pressure lamp comes on while driving, stop the engine immediately. That is not a filter issue but a loss of oil pressure, and carrying on destroys the bearings within minutes.
Common fitting mistakes
Most leaks after an oil change share one cause: the old gasket stuck to the filter mount. The new filter is tightened over it, the seal never forms and oil escapes under pressure. Always check the mating surface before fitting.
Two more mistakes: fitting the new gasket dry — it must be smeared with a thin film of clean oil — and over-tightening with a wrench. A filter screwed on by hand until it seats, plus three quarters of a turn, is enough. On cartridge filters, replace all the O-rings supplied in the kit, not just the paper element.
Indicative change intervals by usage
| Usage pattern | Recommended interval | Why |
|---|---|---|
| Mixed, with long journeys | 12,000–15,000 km or 12 months | The engine runs at optimal temperature and the oil degrades slowly |
| Mostly urban, short trips | 7,000–10,000 km | The engine never warms through; condensation and fuel dilute the oil |
| Dust, construction sites, unpaved roads | 7,000–8,000 km | The filter loads up far more quickly |
| LPG or CNG installation | 7,000–8,000 km | Higher combustion temperatures put extra load on the oil |
| Very low mileage, under 5,000 km a year | Once a year | Oxidation and condensation do not depend on mileage |
How to change the oil filter properly
The essential steps of a by-the-book filter change, whether you do the work yourself or check what the garage did.
- 1
Warm the engine
Let the engine run for 5–10 minutes, then switch it off. Warm oil drains better and carries deposits with it, but wait a few minutes before working so you don't scald yourself.
- 2
Drain the old oil
Undo the drain plug and let the sump empty completely, 10–15 minutes. Replace the plug washer — on most engines it is a single-use part.
- 3
Remove the old filter
Unscrew the filter with a filter wrench and always check whether the rubber gasket stayed stuck to the mount. Wipe the mating surface with a lint-free cloth.
- 4
Prepare the new filter
Smear the new filter's gasket with a thin film of clean oil. On cartridge filters, replace every O-ring supplied in the kit and respect the torque figure printed on the cap.
- 5
Fit it and check
Tighten the filter by hand until it seats, then a further three quarters of a turn. Add the new oil, start the engine, wait for the pressure lamp to go out, and check the level and for leaks after a few minutes.
Frequently asked questions
Can I change only the oil, without the filter?
It is not advisable. The old filter stays loaded with particles and, through the bypass valve, can return them to the fresh oil. The filter costs little compared with the oil, and replacing it adds no labour time.
Does the filter brand matter?
The construction matters: the anti-drainback valve, a correctly calibrated bypass valve and the filtration area. Very cheap filters without an anti-drainback valve let the galleries empty between starts, which means a few seconds of running with insufficient lubrication.
Should the filter be filled with oil before fitting?
Only on filters mounted vertically with the opening upwards, where it can be done without spilling oil on the mount. With horizontal or downward mounting it makes no sense; what matters is waiting for the pressure lamp to go out after starting.
The information in this guide is indicative. Always follow the intervals and specifications given in your own car's owner's manual.
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