Why Do Oil and Filter Changes Matter?

Engine oil reduces friction, carries away heat and holds dirt in suspension, and over time it loses the ability to do all three. A new filter removes the dirt that oil collects, so changing both together keeps the engine protected.

Putting off an oil change rarely causes a problem on the day, but it is one of the most common reasons engines wear faster than they should. Here is what actually happens inside, how to check your oil yourself and when low oil means you should stop driving.

What does engine oil actually do?

Oil forms a thin film between moving metal parts, such as pistons, bearings and the camshaft, so they slide rather than grind. It also takes heat away from places the cooling system cannot reach and suspends tiny particles until the filter traps them.

Additives in the oil neutralise acids from combustion and help prevent sludge. These additives are used up gradually, which is why oil has a limited working life even if the level on the dipstick looks fine.

What happens if I leave an oil change too long?

As oil ages it thickens, picks up soot and fuel and moisture, and loses its protective additives. Diesel engines in particular can build up soot in the oil, and short trips in any engine leave moisture that does not burn off.

The result is more friction, more heat and, in time, sludge that can block narrow oil passages. Wear on bearings and other internal parts is not reversible, so what could have been a routine service can become a major repair.

Why is the oil filter changed at the same time?

The filter traps particles of metal, carbon and dirt as oil flows through it. Once it fills up, it either restricts flow or, in some designs, a bypass valve opens and lets unfiltered oil through.

Fitting new oil into an engine with an old filter would put clean oil through a clogged element, which defeats the purpose. That is why a garage replaces the two together. The cost of an oil change depends on the amount and grade of oil your engine needs, so ask for a quote naming the car.

How do I check my oil level safely?

Park on level ground with the engine off and give it a few minutes to settle if it has been running. Pull out the dipstick, wipe it, push it fully back in and pull it out again to read the level between the minimum and maximum marks.

Some cars have an electronic oil level reading instead of a dipstick, so check the handbook. Topping up with the correct grade is better than running low, but do not overfill. Warning signs to take seriously include:

• The oil pressure warning light coming on while driving
• Oil level at or below the minimum mark
• A burning smell or blue smoke from the exhaust
• Visible oil dripping under the car
• Ticking or knocking noises from the engine

When should I stop driving because of oil?

If the red oil pressure warning light comes on while driving, pull over safely as soon as you can and switch off the engine. Continuing to drive without oil pressure can destroy an engine in a short time.

Check the level once the engine has cooled, but do not restart if it is empty or the light remains on. A garage can then inspect for leaks or faults. Automaster, at Longcombe Cross near Totnes, can inspect petrol and diesel cars, and we never promise a repair until we have looked at it.