Topping up oil — which oil to add between changes?

Topping up oil between changes is one of the most common tasks a driver deals with — especially in older cars or engines that naturally consume a little oil. The situation usually looks the same: the oil level warning light comes on, or a routine dipstick check shows the level has dropped. At that point two things matter: adding the right oil and doing it sensibly, without overdoing it in the other direction. This article explains step by step which oil to choose, how much to add, how to check the level, and why too much oil can be just as dangerous as too little.

It is worth dispelling a myth straight away. Losing oil within reasonable limits does not always mean a fault — even a healthy, modern engine is allowed to consume up to around a litre of oil per thousand kilometres, though in practice most engines use far less. The problem only begins when top-ups become frequent and regular, and the level drops faster than usual. In that case topping up treats the symptom rather than the cause — but even then, keeping the level correct is absolutely essential, because running an engine on too little oil is the quickest route to an expensive repair.

Which oil to add?

Rule number one is simple: it is best to add exactly the same oil that is already in the engine. That means the same product from the same brand, with an identical viscosity and the same manufacturer approval. This guarantees that the top-up blends with the oil already working in the engine without any compromise in properties. That is why many experienced drivers keep a spare litre of exactly the oil they run in the boot — it is the cheapest insurance against a sudden drop in level on the road.

If you do not have the identical product to hand, the key thing is above all matching the viscosity and the specification. Viscosity refers to those symbols like 5W30 or 5W40 — they have to match, because they define how the oil behaves cold and hot. The engine manufacturer's approval is equally important, meaning the standard the oil must meet. Adding an oil with the same viscosity and a similar specification will do no harm, even if it is a different brand. A worse idea is reaching for the first random oil without checking whether it fits your engine at all.

Particular caution is needed in modern diesels with a particulate filter and in engines covered by strict approvals, such as VW, BMW or Mercedes. There, a generic "5W30 oil" is not enough — it has to be an oil meeting a specific Low SAPS standard, because a random product can eventually damage the DPF. In such cars even an emergency top-up should comply with the specification. If you are not sure which standard applies to your car, check the service book or the sticker in the engine bay before adding anything.

Can you mix oils in an emergency?

This question keeps coming back and stirs up a lot of emotion. The answer is: as a one-off, in an emergency, yes — adding a different brand of oil with the same viscosity will not destroy the engine. In practice it is always better to add something sensible than to drive on a dangerously low level, because the latter risks far more serious consequences. Modern engine oils are largely compatible and do not react violently with one another.

You do, however, have to be clear: a blend of two different oils has averaged, usually poorer properties than a single high-quality product. It is also hard to be sure that such a blend still meets the required approval. So treat mixing oils as a rescue to get you to the workshop, not as a permanent solution. At the next scheduled service it is worth going back to a single, correctly chosen oil and replacing it in full along with the filter, so that the engine again runs a product with known, uniform properties.

How much oil to add?

Here an iron rule applies: add a little and check often, rather than pouring in a lot at once. The distance between the lower and upper mark on the dipstick is usually about one litre of oil. That means if the level has dropped just below the lower mark, you are typically about a litre short of full — but never pour the whole litre in blindly. It is far safer to add in small portions, around 100–200 ml at a time, and to recheck the level after each one. That way it is practically impossible to overfill. Keep in mind that engines differ in total oil capacity — some hold three litres, others eight or more — so the same drop below the mark can mean a very different amount missing. When in doubt, add less and measure again rather than trust a round number.

How to add oil step by step

1. Park the car on flat, level ground — a slope will falsify the level reading on the dipstick.
2. Switch off the engine and wait a few minutes for the oil to drain back into the sump and the unit to cool slightly (hot oil and heated parts risk burns).
3. Check the current oil level with the dipstick so you know how much is really missing.
4. Unscrew the oil filler cap, usually marked with an oil-can symbol on top of the engine — do not confuse it with the coolant cap.
5. Add the oil in small portions, around 100–200 ml, ideally through a funnel so you do not spill it.
6. After each portion wait a moment, insert the dipstick and check the level, repeating until you reach the area of the upper mark.
7. Screw the filler cap back on firmly, start the engine for a few seconds, switch it off and check the level one last time after a moment.

How to check the oil level with the dipstick

Measuring correctly with the dipstick is simpler than it seems, but easy to get wrong. The car has to stand on level ground, and the engine should be switched off and preferably slightly cooled. Pull out the dipstick, wipe it clean with a rag or paper towel, push it fully back in and only then pull it out again to read the level. The oil trace should sit between the lower and upper mark — ideally around the middle or closer to the top. If the oil is barely visible at the lower mark, it is time for a top-up.

While you are measuring, pay attention to the oil itself. If it is milky white or has the consistency of mayonnaise, it may indicate the presence of water or coolant — in that case a top-up solves nothing and you have to find the cause. If there is no oil visible on the dipstick at all, do not drive on until you have restored the level. In many newer cars the classic dipstick has been replaced by an electronic sensor and a level reading from the on-board computer menu — then follow the car's manual, because the measurement procedure can differ.

Why must you not overfill the oil?

Many drivers intuitively assume that since too little oil is harmful, more oil must always be better. That is a serious mistake. Too much oil is just as dangerous as too little, only it does harm in a different way. When the level exceeds the upper mark on the dipstick, the spinning crankshaft starts to churn the oil, foaming it up. Foamed oil lubricates worse and loses its properties, which accelerates engine wear. Pressure in the system also rises, which can overload the seals and lead to leaks.

An excessively high oil level can also be drawn by the crankcase breather system into the intake, where it fouls sensors, the EGR valve or the turbocharger. In extreme cases too much oil can cause a diesel engine to run away uncontrollably. That is why the golden rule is: aim for the area of the upper mark, but never exceed it. If you have accidentally overfilled, the excess has to be extracted or drained — you must not brush it off, hoping it will "burn off by itself".

A good top-up is one after which the oil level sits around the upper mark of the dipstick — neither too low, nor a single millimetre above the maximum.

Finally, remember that a regular need for top-ups is always a signal worth taking seriously. If your car calls for oil every few hundred kilometres, the cause may be engine wear, a leak, damaged valve seals or a turbo problem. Topping up will keep the car running, but it does not replace a diagnosis. We will gladly check the condition of your engine during a mobile oil and filter change at the address you choose, selecting an oil that exactly matches your car's specification.

Can you add oil to a hot engine?

Technically yes, but it is not the most convenient. It is better not to look into a hot, freshly switched-off engine straight away — it risks burns, and the level reading will be falsified because some of the oil is still sitting in the upper parts of the unit. It is best to wait several to a dozen or so minutes for the oil to drain into the sump and cool slightly. Only then will the dipstick reading be reliable and the top-up itself safe.

What to do when the oil light comes on?

Tell the two warning lights apart. A yellow low-oil-level light is a reminder to check the dipstick and top up the level. A red oil-pressure light is an alarm — if it comes on while driving, stop safely at once and switch off the engine, because driving on risks seizing it. The red light does not always mean a lack of oil; it can point to a pump failure or another problem that a top-up will not solve.

Do you have to top up with the same brand of oil?

Ideally yes, but it is not an absolute requirement. What matters is matching the viscosity and approval, not the manufacturer's name itself. If you have an oil of a different brand to hand but with an identical viscosity and meeting the same standard, you can safely add it. Sticking to one brand between changes is convenient and removes any doubt, but for an emergency top-up the parameters matter most, not the logo on the can.

How often should you check the oil level?

In a newer, healthy car a check every few fill-ups or once every few weeks is enough. In an older engine, or one that consumes a little oil, it is worth looking at the dipstick more often — even at every fill-up. Always check the level before a longer trip, because driving with a low oil level on the motorway, at high revs and temperatures, is especially dangerous for the engine. Regular checking also lets you catch rising oil consumption early.