A Guide to Safe Engine Lubrication: Can I Mix 5W30 with 5W40 oil?

Like we take our meals to stay fit, engines require proper lubrication to maintain optimal performance. In this case, motor oil is the food for engines to reduce friction. It also prevents metal-to-metal contact between moving parts and further wear and tear. The problem appears when we need to select the right grade of oil. Such questions come to mind – can I mix 5w30 with 5w40 oil grades?

Well, these numbers are all about the oil viscosity alias, the thickness, or resistance to oil flow. It refers to how easily our oil can flow and lubricate the parts. Technically, you can mix 5w30 and 5w40 oils, yet experts don’t recommend doing so (1,2,3). Mixing them can provide the oil with an unpredictable viscosity. It may not provide the proper lubrication for your engine.

Different grades of oil may not mix properly due to their different weights. It may prevent the engine from receiving the full benefits of either oil (4). Still, you can mix and use it in certain situations. But when? To learn more about mixing 5w30 and 5w40 oils, I suggest you continue reading this article until the end.

Motor Oil Grade – What You Must Know

You may have a query – what are the numbers “5”, “30”, “40”, and the letter “W” in oil grades? As I said earlier, oil grade refers to the oil’s viscosity. These combinations of numbers and letters are oil grades. Let me make it simple.

The first (or only) letter, “W,” stands for “winter.” It highlights the oil’s performance in frosty conditions. The number before this “W” shows how the oil can flow in cold starts. It can be any of these numbers: 0, 5, 10, 15, and 20.

Experts say the lower the number, the better the oil flows. In simple terms, lower numbers mean the lubricant will still flow smoothly under lower temperatures without freezing or getting thicker. In this case, “0” is the best and thinnest type of oil grade.

Now ask me, what about the latter numbers? The numbers following the “W” (i.e., 30 and 40) represent the oil’s viscosity at its operating temperature, typically 100°C (212°F). In another sense, they refer to the oil flow at higher engine temperatures (5). It is the opposite of “5” before “W.”

Generally, ’30’ is often used for newer petrol engines. On the other hand, ’40’ is often used for older or diesel engines (6).

The higher the numbers after “W”, the oil will resist the engine’s heat during summer days. So, 5w40 will take more time to get hot than 5w30. This oil grade is better if you mostly live in an area with high temperatures.

Again, another question: which oil will you choose? Or can you mix them to defeat odd situations? Do not worry about these things, as I will make it clear in the rest of this article.

Differences Between 5w30 and 5w40 Oils

Well, the difference between 5w30 and 5w40 oil viscosity ratings lies in their working viscosity.

5w30 oil has a working viscosity of 9.3 to 12.5mm2/s, while this viscosity for 5w40 is 12.5 to 16.3mm2/s. This means that 5w40 oil has more viscosity than 5w30. Also, both motor lubricants have a winter viscosity rating of 5. They are effective even at -30°C. In short, 5w30 will be the best option for you for smooth flow and quick lubrication (7, 8, 9).

Properties of 5w30 Oil

5W-30 oil is a blend of base oils composed of petroleum-based hydrocarbons, polyalphaolefins (PAO), or their mixtures in various proportions. Sometimes it can be with up to 20% by weight of esters for better dissolution of additives (10).

One of the specific chemicals found in 5W-30 oil is Zinc C1-C14 Alkyldithiophosphate (CAS No. 68649-42-3) (11). Another chemical component is Phosphorodithioic acid, OO-di-C1-14-alkyl esters, and zinc salts (CAS number 68649-42-3). It makes up 0.8 – < 2% by weight (12). The oil’s specific gravity at 60°F (15.6°C) is 0.860, with a density of 7.17 lb/gal (13).

Properties of 5w40 Oil

5w40 can be composed of a lubricant base stock. Synthetic oils, like 5w40, often use poly-alpha-olefin (PAO) as a base stock in their production (14). It can include additives to improve the oil’s detergency, extreme pressure performance, and ability to inhibit corrosion of engine parts (10).

One of the components in 5w40 oil can be alkarylamine. It is a hazardous substance (15). For sure, the chemical composition of the oil can influence its specific gravity. It can change with the increasing content of aromatic hydrocarbons. Also, it can decrease with the increasing content of saturated hydrocarbons (16).

Can You Mix 5w30 with 5w40 Oils?

So far, I have given you the ideas for both oils. You have already understood that these oils are not the same in their quantities of ingredients. Now, think of a situation: you have both oils and want to mix them. Is it possible?

Well, the answer is not a simple “yes” or “no.” You must understand the complexities and solve if you can mix them actually.

As I explained earlier, the 5w30 oil is generally preferred for engines requiring less bulky or thin viscosity. It is often used in colder climates. Also, it is more fuel-efficient compared to 5w40 oil (17). Alternatively, 5w40 oil is typically used in temperatures up to 40°C.

The Consequences of Mixing Oils

The 5w40 oil is meant for higher temperatures. It will not deliver its full potential when mixed with 5w30. Mixing these oils improperly may or may not damage your vehicle’s engine (18). If the oils have different viscosities, it can cause issues like reduced fuel efficiency and degraded performance. It may make you spend more money at the pump and potentially increase emissions.

Right now, manufacturers provide engine oils containing various additives to enhance performance and protection. When you mix oils, the incompatible additives may interact negatively and compromise their functionalities. In worst-case scenarios, incompatible additives can react to form sludge. It can clog engine passages and harm vital components.

When we buy a car, manufacturers give us warranty terms for it. Suppose we use non-recommended oils, including mixtures. It may void the warranty. Again, it is related to our expenses. After all, we should consider factors like climate, the type of engine, and vehicle model when choosing (or mixing) oils. Okay, let’s leave the negative things.

Still, many experts say that mixing 5w30 with 5w40 may not cause any adverse effects on your engine (19). Both oils will flow evenly at low temperatures into all engine components. Certainly, it is beneficial for fuel economy (1). However, when mixed, the engine will not fully enjoy the potentials of both oils.

When the Mix Becomes (Reluctantly) Necessary

Keeping one thing in mind (least damage to the engine), sometimes mixing two different oil grades is permissible. Let’s discuss the situations:

Let’s think – you are miles away from the nearest fuel-filling station. You need to refill the lubricant to reach your destination. In such emergencies, you can mix a small amount (like up to 1 quart) of 5w30 and 5w40. To me, it works like a temporary solution.

If you must mix, you should minimize the risk by ensuring both oils share similar specifications or brands (2). I suggest you pick both oils of similar types (like synthetic). For better efficiency, they must meet the same API standards (for instance, APN SN).

Final Words: Can I Mix 5w30 with 5w40?

So, you can see, you can mix oils in some cases, like emergencies. It may keep your engine running, yet it can be disastrous to your engine. However, you need to consider the suggestions I have provided.

You may also consider some alternatives. Multi-viscosity oils can be one solution (20). These oils have polymers added to a light base. It prevents the oil from becoming thinner as much as it warms up. Hence, the polymers are coiled up at low temperatures. Then, they allow the oil to flow as their low numbers indicate.

The polymers unwind into long chains as the oil warms up. They resist the oil from thinning out like it usually would. The result is that at 100°C, the oil thins only as much as the higher viscosity number indicates. It allows the oil to behave like a lower-viscosity oil in colder temperatures. At the same time, it still performs the same as a higher-viscosity oil at operating temperature.

Other alternative ways can be regular oil changes and maintenance. Even if you use a mixture, regularly changing to a fresh, particular (recommended) grade of oil may make the engine perform better.

That’s all for today! I hope you enjoyed reading this article. Thank you.

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