How often should motor oil be changed?

Why time counts just as much as mileage

Oil does not age through friction alone. It oxidises under oxygen and heat, it absorbs combustion residues, and the additives get used up: detergents keep soot in suspension until their capacity is exhausted, and the base reserve neutralises acids until it is spent. Condensation collects above all in engines that rarely get warm, and forms acids together with combustion gases. A vehicle that has hardly been driven for a year therefore has oil in the engine that has aged chemically, even though the odometer has barely moved.

Severe conditions: when to change earlier

The manufacturers themselves list in their maintenance schedules the conditions under which the interval is shortened. In practice these are:

• Regular trips of under about ten kilometres, on which the engine does not reliably reach operating temperature.
• Frequent cold starts at low outside temperatures.
• Towing trailers and caravans, roof loads, constant full load.
• Dust, sand and building-site surroundings, because more dirt gets in.
• Stop-and-go traffic, taxi, courier and driving-school use: many operating hours over little distance.
• Long periods standing idle, as with seasonal vehicles and classic cars.

What the service indicator actually measures

The flexible service indicator of modern vehicles does not just count kilometres. It weighs up operating hours, engine speed levels, oil temperature, the share of cold starts and, depending on the model, the signal from an oil quality sensor. That is why the service falls due at very different mileages on two identical vehicles. The indicator is reliable as long as the precondition holds: the oil poured in must have the approval the vehicle is counting on. More on this in the article Longlife oil or a fixed change interval.

Seasonal vehicles and long periods off the road

For vehicles that stand idle over the winter, fresh oil before the lay-up is better than fresh oil after it. Used oil contains acids and water, and both attack bearing surfaces over the months in which the engine does not run. If you were going to change the oil in spring anyway, you are better off moving the change to the autumn.

Topping up is not an oil change

Engines that consume oil are topped up with fresh oil again and again over their life. That does not replace the change: the contaminants and the spent additives stay in circulation, only the quantity is right again. Topping up calls for the same specification as the fill, otherwise different additive packages get mixed. You will find suitable packs in every size in the motor oil range, and larger quantities for the workshop under 20-litre containers.

How to tell the oil is due

Colour is no yardstick. A diesel oil is black after a few hundred kilometres because the additives keep the soot in suspension; that is exactly their job. More telling are a clearly risen oil level, which points to fuel dilution, a milky film on the oil filler cap on short-trip vehicles, and the date of the last change in the service book. Anyone who wants to know for sure sends an oil sample for laboratory analysis; in fleets this is an established way of proving intervals rather than guessing them.

Planning oil and filters together

The oil filter is replaced at every oil change, the air filter and the cabin filter usually on a longer cycle. Combining the two saves an appointment. How the change is done is in the oil change guide; returning the used oil is covered on the Waste oil returns page.