It is very common for car drivers to experiment with different oil grades in their cars, especially older ones. However, is that safe? Let’s analyze this topic by considering two very common oil grades, the 5W30 and the 5W40
Can I Put 5W40 Instead of 5W30?
In most cases, yes. The codes indicate that both oils have the same viscosity at low temperatures, while the first one is somewhat thicker at higher temperatures. Since the difference is not so big, it usually falls within the range recommended for most applications.
Then again, that still means you are not using the most appropriate oil type, so it is recommendable to replace it as soon as possible. Since there are cost implications in the matter, a good compromise is to anticipate the next oil change by a thousand miles or so.
In practice, many drivers like to fiddle with slightly different oil grades to see how the car’s performance is affected. That is harmful to the engine and particularly problematic if the car is still covered by the factory warranty, because it may be interpreted as leaving the original conditions in which the car is supposed to be used.
An immediate replacement would only be necessary if the engine’s requirements are highly specific and/or the wrong oil applied is excessively different from the recommended one. Repairing the damage it would probably cause would be much more expensive than quickly replacing the oil.
Also read: Does Engine Oil Brand Matter? Can You Buy a Random Brand?
What’s the Difference Between 5W40 and 5W30?
The difference is that the 5W40 is thicker at high temperatures. The 5W40 has a viscosity range of 12.5 to 16.3 mm²/s while the 5W30’s range is 9.3 to 12.5 mm²/s. In other words, the 5W30 is more affected by high temperatures; it becomes fluid at a higher rate than the 5W40.
In general, when the oil is too viscous for the engine, it resists more flowing through its circuit, so it demands more energy from the pump. Besides, it will not penetrate every required cavity as it is expected to ensure the proper operation of the engine.
On the other hand, having an excessively fluid oil means that its whole lubricant action will be minimized. The oil will not be able to execute its role as much as it should and the engine will end up with excessive friction anyway – and all the problems that come from it, such as too much noise and vibration and more intense wear due to grinding.
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High viscosity at high temperatures is desirable for engines that work under heavy load, whereas high fluidity at high temperatures is interesting in case the engine has many cavities that require lubrication and if they are of precise dimensions. Then again, the difference between 5W30 and 5W40 is not so big.
What Bad Will Happen if You Put 5W40 Instead of 5W30?
The engine will be lubricated with a thicker oil than expected. In short, the oil will not be able to flow as it is required, which means that it will reach the necessary regions more slowly and/or in smaller quantities and, in extreme cases, may not even reach some regions at all.
Since the oil is thicker than expected, the oil pump will demand more energy to do its job. There would be a higher demand for the engine and a subsequent increase in fuel consumption.
Another issue is that the oil’s poor flow will leave several regions with low or even no lubrication at all. It is possible for those regions to grind, generate excessive noise and vibration, and even grind. That would contaminate the oil with residual particles and accelerate the engine’s wear.
Also read: 4 Bad Things that Will Happen if You Don’t Change Motor Oil
Could Putting 5W40 Instead of 5W30 Be Good in Any Way?
In theory, a thicker oil performs better at high temperatures and high workloads. Therefore, one could imply that it would be interesting to use the 5W40 in case you are planning to make more intense use of your car. However, that is more risky than useful.
Modern engines, especially those used in cars, are manufactured with increasing precision in order to yield higher performance. Therefore, it is normal for all manufacturing criteria to become stricter to fulfill their requirements year after year. Choosing a different oil grade on your own, especially outside the recommended range, is definitely not worth the risks you would take.
Are There Any Situations When You Can Put 5W40 Instead of 5W30?
The only situation where that is safe to do is when the engine manufacturer allows it. In general, that will be expressed as a range of acceptable oil grades.
Every engine has a single oil grade that works perfectly in it; anything thicker would not be able to reach all the necessary regions appropriately, while more fluid oils would not withstand the regular workload as expected.
On the other hand, it is necessary to observe that these effects are gradual. Basically, all engines accept some deviation from that “perfect grade” up to a certain point where the difference in the oil’s action becomes harmful. In other words, all engines have an acceptable range of oil grades.
That is a favorable situation because there will always be manufacturing variance between samples of oil. In fact, each code name like 5W30 represents a range of expected viscosity rather than one single value.
In practice, the more precise the engine’s manufacturing and work demand are, the narrower that acceptable range will be – in some cases, that reaches the point of admitting only one oil grade. However, it is difficult to find an engine that accepts many oil grades because that would require highly imprecise manufacturing, which would affect its efficiency.