Can you add oil to a hot engine?

As an automotive engineer, and working in many departments, I often received this question from customers. Is it safe to add oil if the engine is hot? Let’s find out in this article.

Although it is best to add oil when the engine is cold, nothing bad will happen to the engine if you add oil to a hot engine. The metal parts of the engine are not so weak that something bad will happen to them if colder oil is added to the engine, the most important aspect is that the oil is the same as the one already in the engine, because it is not exactly indicated to mix the different viscosities of the oil, except under certain conditions.

When to add oil? In a cold or hot engine?

To begin with, adding oil is often done when the engine is cold because it is easier and faster to determine the required level, to avoid possible underfilling or overfilling with oil.

When temperatures are colder, especially in winter, it is best to first warm up the engine, then give it time to cool down no more than 15-20 minutes if temperatures are below freezing point, to let the oil settle. This preheating will allow the heavily thickened lubricant to return to its proper fluidity.

After that operation, the oil level can be determined quite accurately. Next, the newly fresh oil is easily mixed with the one existing in the engine into a homogeneous mass. Be careful what type of oil you add, it should be the same oil.

However, you might encounter emergencies when have to add oil to a hot engine, for example, when the emergency oil pressure light comes on the dashboard while driving. This situation is quite common and becomes a reason for questions like what will happen if you add cold oil to a hot engine.

You saw in the dashboard that oil is below the minimum level, or stopped the car and checked the dipstick and found out that the oil level is low. Then you immediately took a canister from the trunk and refilled it until you were certain that it is safe to drive again. 

When the engine is hot, there is a risk that when you refill with oil, you might add too much, and this overfill will not do the engine any good. The risk increases in summer when temperatures are high and the engine cools down slower. But in winter things are a bit different.

That’s why it is better to wait at least 15 minutes for the engine to cool down and only then add oil.

It is not difficult to guess that the oil storage in the trunk will be cold, but when pouring into a warm engine, a temperature difference will occur, but engine oil absorbs heat at a much slower rate than coolant so the cooling will not be sudden.

Some say that cracks might appear in the block, head, or other engine parts, as well as other faults in relation to individual elements of the internal combustion engine. This is not true, the only important two aspects you should watch for are overfilling and mixing different engine oil viscosities.

Adding oil to a car engine. Step-by-step instructions for beginners

  • So, after making sure of the need for topping up and deciding on the right type of oil you will add to the engine, you will need to put the car on a level surface.
  • Then let the engine cool down, it is advisable to leave the car for some time, and also let the oil drain completely into the crankcase.
  • Now you will oil filler neck or another tool like a funnel to make sure the oil doesn’t reach other parts of the engine. The specified tool is, usually, located under the cover, at the top of the cylinder head. Most often, the lid has a pictogram in the form of an oiler with an icon that resembles a drop of water
  • Next, unscrew the lid, you can also wipe it with a clean cloth and then set it aside.
  • Add the oil filling tool or funnel, or if you don’t have one, cut the upper part of a plastic bottle, this will be enough.

The presence of the tool allows you to accurately add oil, without the risk of spilling lubricant on the cylinder block and cylinder head. Oil on these parts will result in its subsequent burnout from high heat, smoke, and unpleasant odors.

Also, engine oil droplets are not that good on rubber elements, on soft insulation, and on all kinds of seals and similar elements in the engine compartment. If oil has nevertheless been spilled, then it is recommended to thoroughly wipe it off.

  • Adding oil should be done gradually. This means that 100-200 ml should be poured from the canister at a time. Next, you need to allow it to drain from the cylinder head into the crankcase. This may take about 10-15 minutes. Then the level is checked, after which you can continue to add oil if necessary.
  • When checking the level on the dipstick, you must first remove the dipstick, then wipe it with a clean cloth, then re-insert it all the way into the hole and remove it again. Repeat that procedure 2-3 times to make sure it is the correct level.
  • After the oil level on the dipstick is strictly between the “MIN” and “MAX” marks, it is necessary to firmly insert it into the hole and put back the oil filler cap.
  • The final stage will be starting the engine. Evaluate the operation of the internal combustion engine for extraneous noises, knocks, vibrations. Make sure that the oil pressure light on the dashboard does not light up, or it does not show an insufficient amount of oil.
  • Next, warm up the engine, and make a test drive. After that, it is recommended to let the engine cool down, after which the oil level is checked again. If a decrease in the level is again noticeable, refill it and, as an ultimate solution, check to see if you have leaks.


Remember, driving with low oil levels can quickly damage your engine. In extreme situations, refill until you go to a repair shop and check the problem. If the oil leaks are intense, then it is best to use a tow truck.

Image by: Dvortygirl, CC BY-SA 3.0, via Wikimedia Commons

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