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What happens if you put the wrong oil in your car?

Tom Vondrasek | 6th May 2023 | 5 minutes to read

Using the wrong oil in car engines can be hard to identify. If engine oil is in there it will still lubricate the moving parts though not as effectively as the correct oil. It also depends on whether it was a complete oil change or if you just topped it up.

There have been cases where people have forgotten to refill the sump and driven off ignoring the oil light on the dash. This does not end well for their car’s engine.

We’ve put together information on some of the more common instances where the wrong oil ends up in your engine and how to resolve the problem:

  1. Wrong Oil Used During a Service or Complete Oil Change
  2. Wrong Oil Used During a Top Up
  3. Symptoms To Watch Out For
  4. What Happens When You Mix Oil Viscosities?
  5. What Happens When You Mix Synthetic and Conventional Oils?
  6. What Happens When You Mix Different Oil Brands?
  7. What Should I Do To Fix It?

Using the wrong oil in car engines can be hard to identify. If engine oil is in there it will still lubricate the moving parts though not as effectively as the correct oil. It also depends on whether it was a complete oil change or if you just topped it up.

There have been cases where people have forgotten to refill the sump and driven off ignoring the oil light on the dash. This does not end well for their car’s engine.

We’ve put together information on some of the more common instances where the wrong oil ends up in your engine and how to resolve the problem:

Wrong Oil Used During a Service or Complete Oil Change

This does depend on how close to the specs the new oil used is compared to the manufacturer’s recommendations. Oil type and viscosity is the issue here and different people will give you different answers on this point.

As an example, let’s say the oil’s viscosity was the same and you used a semi-synthetic oil instead of a mineral. This is a lot better than if you put a mineral oil in a full synthetic engine and both viscosities were different. In the first case this is probably ok, where in the second, draining it and refilling with the correct oil may be the best solution.

The one point most people agree on is if it is not recommended, then change it to something that is. The longer the incorrect oil is in there the more damage it can do. Sometimes it will take time before you notice something is not quite right, which can mean major damage is already done. This is the reason they put large oil viscosity numbers on the outside of the oil bottles, to make sure that you pick the correct one.

Wrong Oil Used During a Top Up

This is not so bad and, depending on the quantity you added, can probably wait until the next service or oil change to handle the problem. Oil type and brand is not so critical where viscosity may be an issue. Consult an expert and they can best advise on a course of action.

When choosing an oil, always stick with the manufacturer’s recommendations. Companies like Repco & NAPA get the vehicle data from oil companies like Penrite or Castrol who get their information from the manufacturer. It is a way of ensuring you put the correct or recommended oil in your sump.

If you suspect the oil is incorrect look to replace it as soon as possible. Ideally before you drive the vehicle again, especially if it seems to be causing problems.

Symptoms To Watch Out For

Puddle of motor oil under a white car

Reduced Engine Performance

Putting a thicker oil in an engine that needs a thin oil can cause additional friction, slowing up the rotating mass of the engine. The throttle response may be more sluggish than normal. Putting thin oil in a thick oil engine may cause the engine to perform better in throttle response, though it may sound noisier. The reason this happens is that the lubrication between surfaces is not as effective.

Engine Overheating

Thicker viscosity oil does not flow as smoothly, which means it struggles to get into where it is needed in an engine that requires thin oil. This means engine parts may not be as well lubricated and generate more friction which causes heat. The heat builds and some of the oil may even get burnt. If this happens, you’re probably going to smell it. The opposite happens in a thick oil engine with thin oil. The tolerances are larger and the oil drains away too quickly, causing the same issue — including burning the oil.

Oil Leaks

Thin oil in a thick oil engine may weep past seals designed for a thicker oil. The gaps between parts are larger which means the thin oil puts more pressure on existing seals. It is not so bad when thick oil is put in a thin oil engine as the seals are designed for a more fluid oil.

Reduced fuel Economy

With thick oil on a thin oil engine the effects of additional friction on the rotating mass and poor lubrication generating additional heat will cause poor fuel economy. In a thick oil engine, the thin oil has the heat issue as well as being noisier. The fuel economy won’t drop off as much as in the first case so will not be as noticeable.

Increased Wear & Tear

Whether you have the thin oil in the thick oil engine or vice versa, the effects on wear and tear will be similar. The oil is not doing its job correctly so wear and tear will automatically increase. If lubrication is bad enough some components may seize due to heat and lack of oil leading to failures in the engine. If racing or under heavy load the effect will be more pronounced.

As you can see these symptoms may initially not be very prominent, so trying to equate them with an incorrect choice of oil may not be readily apparent. What can be guaranteed is that symptoms should start getting worse the longer you leave it.

The best and easiest course of action is to make sure you use the correct oil in the first place, so take your time when choosing and don’t rush it.

What Happens When You Mix Oil Viscosities?

This is definitely not recommended, as viscosities indicate how thick or thin an oil is and having two different kinds can cause multiple problems. Most of the symptoms listed above come from adding oils of the incorrect viscosity. Mixing different oil viscosities should only be used as a stop gap or in an emergency. The reason is there are so many variables you need to write a book of rules on it.

For example:

  • Ratio - What is the ratio of the mix. Your engine only has 25% of the original oil and you need to add 75% of a different viscosity.
  • Are both viscosity grades different or just one of them? Is the winter number the same but the hot number is different, such as with a 5W30 and a 5W40, or are all the numbers different?
  • What is the viscosity difference. Does it vary a lot? For instance, 0W20 to 10W50.

These examples only covered full synthetics. If I went from a synthetic to a mineral, then the gap in viscosity is even greater. There is no doubt that in some cases it would work OK, while in others you could run into serious issues.

What Happens When You Mix Synthetic and Conventional Oils?

This is not recommended as you need to write another rulebook detailing all the variables. It is not as detrimental as viscosities as the assumption is the viscosities match, it is simply the type of oil that is changing. The good thing is there are not many full synthetic oils that match viscosities with conventional mineral oils.

Semi-synthetic oils can work with a mineral oil and some full synthetic oils.

Same as before, unless it is an emergency or stop gap, why do it? When trying to figure out which oil your car takes, stick with the manufacturer’s recommendation as that always gives the best results.

What Happens When You Mix Different Oil Brands?

Again, this is not recommended unless it is a stop gap or emergency. This is due to the additives oil companies place in their oils. These vary in type and quantity and there are approximately a dozen or more in each oil. They can cancel each or out or maybe multiply the effect in the oil. Oil companies are happy to publish viscosities but because additives are their own internal IP, you’re unlikely to find information on them as they don’t want their competitors to know.

The best way to do it if you want to change brands is at an oil change. Completely drain Brand X oil from your engine and replace with Brand Y. You should be changing your oil filter as well as it can hold a fair amount of oil. This is assuming that brand Y is recommended for your vehicle of course.

What Should I Do To Fix It?

So, you have put the wrong engine oil in the car and you need to know what to do about it. As explained earlier, if it is during a top up it is not so bad, depending on the quantity you added. During an oil change or service where the whole lot has been swapped out, the best thing to do is change it as soon as it is practical to do so.

The only saving grace is any oil is better than no oil, so even if it is wrong and corrected quickly the impact will be minimal.

If you’ve put the wrong oil in and you’re needing to replace it, head into a Repco store to get a fresh batch of the right stuff, and some friendly advice along with it.

Check out our range of engine oils

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