How often to change the oil, when mileage is not enough

The service schedule says, for example, "every so many kilometres or 12 months". The second part is not a formality: on many cars used in town it is the condition that comes first. The right intervals for your car are in the service book.

Why time counts

Oil does not just get used up: it gets contaminated. At every cold start a little unburnt fuel and condensation water end up in the sump. With a warm engine they evaporate; on short trips the engine never reaches temperature and they stay there.

The result is that oil used for short trips degrades sooner than oil that has spent the same months on the motorway. If you only drive a few kilometres at a time, do not stretch the interval: many manufacturers give a tighter service schedule for severe use.

What the numbers on the bottle say

`5W-30` is two viscosities, not one.

• The 5W (W stands for winter) tells you how it behaves when cold: the lower it is, the better it flows in the first seconds after starting, when the engine is least protected.
• The 30 tells you how much body it keeps when hot, that is, whether it holds the oil film when the engine is working.

The right viscosity is specified by the manufacturer: an oil other than the one prescribed does not cause immediate damage, but it protects less well.

The code that matters most, and that hardly anyone looks at

Below the viscosity is the specification: codes such as ACEA C3 or the manufacturers' approvals (VW 504.00/507.00, MB 229.51, BMW LL-04). This is not a suggestion, it is a requirement.

Many diesels with a particulate filter need a *low SAPS* oil, that is, low in ash: an unsuitable oil can clog the filter over time, and repairing the filter costs far more than the oil.

If the specification required by the manufacturer is not on the bottle, that oil is not right for that engine, even if the viscosity matches.

Topping up: when yes and when no

An engine that uses a little oil is not necessarily unwell: many manufacturers state an acceptable consumption, and you will find the value in the handbook. Top up with the same oil, or at least with the same specification and the same viscosity.

Mixing two oils with different specifications does not give an average: it gives an oil that no longer meets either one.

The oil filter, which is always changed

There is no oil change without a filter change. The filter holds back wear particles, and when it is saturated a safety valve opens and sends unfiltered oil to the engine. From then on the filter is there, but it does not filter.

The three things to check when you change it

1. The drain plug seal. It is a washer costing a few cents and it is replaced every time: it is made to deform only once.
2. The tightening torque of the plug and the filter: the one given by the manufacturer. Too tight and you damage the thread in the sump.
3. The level, on flat ground and as the handbook says: with the engine cold or a few minutes after switching off, depending on the manufacturer.

This advice is general: for the values and procedures for your car, refer to the manufacturer's handbook.