SAE 30 vs 15w40: What’s the Difference?

Going into your local hardware or auto parts store and glancing over at the shelf with all the different oil types can be super overwhelming. Even if you have a good idea of the type of oil that you want, you might start second-guessing yourself just because of how many options there are. However, once you take a minute to learn how motor oils are categorized things get a lot simpler.

Between SAE 30 vs 15w40 specifically, there are a few pretty important differences. The name of each oil represents a grade that tells you where and when it should be used. Let’s take a look at each in more detail.

SAE 30 vs 15w40 (The Short Version)

When comparing SAE 30 vs 15w40 it is really important to note that SAE 30 oil is a single grade oil, and 15w40 is a multigrade oil. Also, because these two oils do not share a common rating, their viscosity (thickness) is not going to be the same. SAE 30 is commonly used in small gas engines and 15w40 is more often used in heavy-duty diesel engines.

Is SAE 30 the Same as 15w40?

No, it is not. Again, SAE 30 is a single grade oil meaning it has a viscosity that is only rated one time: 30 in hot conditions. 15w40 oil on the other hand is a multi-grade oil. Its low temperature or “w” (winter) rating is 15, and its high-temperature rating is 40. The Society of Automotive Engineers (SAE) uses a scale from 0-60 to measure the viscosity of oil. Lower numbers represent lower viscosity ratings meaning that the oil will be thinner. Oppositely, higher numbers represent higher viscosities and thicker oils. Multi-grade oils are made with additives that allow the oil to remain thin and very fluid at low temperatures. This is very important when trying to start an engine in cold climates.

The Characteristics of SAE 30 and Its Benefits

For a long time, SAE 30 has been known as the king of small engine oils. With a viscosity that sits right in the middle of the chart, it is able to be used in all kinds of equipment that is powered by small motors. Part of the reason for this is that it’s a single-grade oil that doesn’t cost very much and is easy to come by. Multi-grade oils and their additives that alter viscosity depending on temperature haven’t been around as long as single-grade oil.

Having stood the test of time, SAE 30 is a great choice for older gas engines and ones that don’t operate in the winter. This is often the case for lawn mowers. Even in some of the colder climates where grass grows, you usually won’t run into very low temperatures when you’re mowing the lawn. Single-grade oils like SAE 30 also tend to maintain their original viscosity and last a little bit longer.

The Characteristics of 15w40 and Its Benefits

Like all multi-grade oils, 15w40 has the advantage of being able to adapt to hot and cold temperatures. This makes it really useful for machinery and engines that operate in all kinds of climates. However, with higher than average viscosity ratings, 15w40 is mainly used in diesel engines like those on over-the-road trucks, tractors, or turbo-charged vehicles. The thickness of this oil makes it a good fit for these heavy-duty engines.

When it comes to lawn mowers, 15w40 is not an oil that I would recommend using. Since you won’t need to worry about extremely cold temperatures while mowing, the multi-grade benefits of 15w40 won’t apply. Also, most 15w40 oil has higher levels of detergents which clean engine parts as the oil cycles through. On a lawn mower, this just isn’t necessary.

FAQs About These Oils

There are a lot of common questions when it comes to SAE 30 vs 15w40, but in the context of lawn care, I’ve found a couple that I see most often. Since I’ve already compared the two oils above, I’ll try and get straight to the point below.

Can I Use SAE 30 Instead of 15w40 in My Lawn Mower?

Though I would recommend checking the manufacturer’s guidelines first, using SAE 30 in a lawn mower is almost always a safe bet. SAE 30 has been the most common small engine oil for a long time and has been proven to work. Compared to 15w40, using SAE 30 is probably a better choice for your mower. Because 15w40 is designed for diesel engines, its properties aren’t well suited for lawn mowers.

Is SAE 30 Thicker Than 15w40?

This is a bit of a tricky question, but in the context of a lawn mower, SAE 30 will not be thicker than 15w40. To give you an answer in more detail, let’s look at the two oils in a few different situations.

First, at room temperature, SAE 30 and 15w40 will have a pretty similar thickness or viscosity. This would be a good representation of the state of the oil when first starting a lawn mower.

Next, while hot, 15w40 will be thicker or have a higher viscosity than SAE 30. This is shown by the rating of 40 versus 30 between the two oils. When the engine is warmed up, SAE 30 will flow more easily than 15w40 would.

Lastly, and where it gets a bit complicated, is at cold temperatures. This is not very relevant when it comes to lawn mowers since you probably won’t be mowing in freezing temperatures, but it is worth noting that SAE 30 will be thicker than 15w40 when cold. This is because 15w40 is a multi-grade oil with additives that lower its viscosity when cooled. The reason for this is because vehicles and other diesel equipment need to operate in many different climates, and very thick oil makes cold starting difficult and puts strain on the engine.