What Type of Oil Does a Generator Use? (Portable & RV Units)

generator oil type

Disclaimer: As this is an affiliate site, I’ll earn a small commission from qualified purchases, at no additional cost to you.

The engine oil of a generator helps lubricate the moving parts of the engine. It protects the engine from corrosion and prevents damage from friction. It also keeps the engine clean and cool.

In short, the oil in the generator’s engine ensures proper functionality, protects it from damage, and increases longevity. However, not all types of engine oil are suited to work with your generator.

So, what type of oil does a generator use?

The operating temperature is the most important factor in determining the type of oil to use in a generator. Using a generator in colder climates requires thinner oil or oil that is less viscous. On the other hand, generators running in hotter climates will require thicker oil or oil that is more viscous. 

However, other factors play a role in determining the type of oil to be used in a generator.

In the post, we’ll talk about the various factors to determine the right generator oil type. We’ll also talk about the best type of oil for both portable and onboard RV generators.

But first, let’s look at the type of generator oil and their grades.

Generator Oil Type

While many of the motor oil types used in cars can be used safely to run generators, there are two main types of motor oil widely used and recommended by generator manufacturers.

Synthetic Oil

Synthetic oil is a man-made oil that is engineered to produce high-quality motor oil. It enhances the engine protection level, cleans the engine better, and is much pure compared to conventional oil.

Synthetic motor oil also works better in both low and high-temperature extremes. Though synthetic oil costs significantly more compared to regular oil but is designed to last longer.

The synthetic blend version of the motor oil is a mixture of synthetic and conventional oil that is used by many generator owners. Though the synthetic blend motor oil provides almost the same level of protection but costs less compared to full synthetic oil.

Synthetic oil is best for new generators but shouldn’t be used with older units.

Conventional Oil

Conventional or regular oil is produced by refining crude oil, making it easy to produce and most importantly, budget-friendly. Because of its production process, conventional oil is not very refined, affecting the level of protection and longevity.

Despite the drawbacks, conventional motor oil has its place. This is because not all generators are designed to work with synthetic oil, especially older units that have 100’s hours of runtime under their belt.

In short, synthetic motor oil is best for new generators that will be used regularly and put on 100’s of hours of use. On the other hand, conventional motor oil is best for older generators that have worn out engines.

Generator Oil Viscosity Grades

Motor oil manufacturers use a rating system designed by the Society of Automotive Engineers (SAE) to determine the viscosity (aka grade or weight) of the oil. The viscosity rating of motor oil helps differentiate the type and application of the oil.

The viscosity of fluid measures how easily the liquid can flow. The higher the viscosity, the more resistance the liquid will face while flowing, and vice versa. For example, the viscosity of honey is higher than water making honey a thicker liquid than water.

The viscosity grade of the oil determines the performance of a specific grade of oil at operating temperature.

You may have seen a combination of numbers and a letter such as 5W-30, 10W30, or 30-weight on motor oil bottles. These numbers represent the various grades of oil. The higher the number, the thicker the oil, and the low the number, the thinner the oil.

The Difference between Monograde & Multigrade Oil

Monograde or single-grade motor oil has static viscosity. Meaning the thickness of the oil is not affected by outside or generator operating temperature. A single grade SAE40 oil has a higher viscosity than a SAE30 oil.

But your generator operates at various temperatures depending on the climate and load. The outside temperature doesn’t matter a lot when the unit is running as both the engine and oil are hot.

However, it’s an entirely different thing when you’re trying to start your generator after a cold or hot night. This is because oil becomes thick as it cools down and thin as it warms up, affecting the startup and operation of the engine.

The problem with monograde oil is the viscosity will remain constant irrespective of the temperature. For example, a 30 or 40-grade oil is thick over a cold night it becomes thicker, making engine startup very difficult.

This brings the need for multigrade or multi-weight oil.

Multigrade motor oil uses advanced technology to produce oil that can vary the viscosity of the oil depending on the temperature. Multigrade oils are denoted by a mix of numbers and a letter like 5W20 or 10W30.

The number before the “W” is the actual weight of the oil and the second number represents the weight it can attain as the temperature increases or gets hot. The “W” simply represents winter.

Every generator manufacturer has a recommendation on the weight of oil to be used with their models. Some recommend a single-grade oil, whereas others are multigrade. However, the grade of generator oil you should use will be determined by the start-up and operating temperature of your generator.

In short, the higher temperature calls for higher-grade oil, and the lower temperature calls for lower-grade oil.

Read More: Can I use 5W30 instead of 10W30 in my generator?

What Type of Oil do Portable Generators Use?

There are a wide variety of portable generator brands that offer a range of models depending on the power output. The top generator manufacturers have their brand of motor oil to be used with their generators.

If you own a portable generator from brands like Honda, Yamaha, Generac, Briggs & Stratton, etc. you can use their brand of oil. However, for other brands of generators, you need to consider the operating temperature, the age of the machine, and your budget to determine the best oil type.

For beginners, I would advise you to stick to the manufacturer’s recommendation when choosing generator oil. However, there are other top-to-the-line motor oils like Castrol, Shell, Mobil 1, Valvoline, etc. that have proven records for improving generator performance and longevity.

What Kind of Oil does Onan RV Generator Use?

Though you need to choose a generator oil type depending on mainly the temperature, the type of fuel a generator uses also plays a role. Because the type of fuel a generator uses changes the total heat the engine produces.

Onan makes RV generators that can run on either gasoline, diesel, or propane, depending on the type you choose. This makes it more important to choose the right type of oil depending on the type of fuel your RV generator runs on.

However, irrespective of the type of fuel your generator use, you can use any motor oil from the top brands, given it meets your generator’s specification. You need to refer to your generator’s user manual to find the specs.

Many RV owners have found great results using the Mobil 1 oil in their Onan generators. Make sure to take into account the operating temperature when deciding the grade of oil.

Whether you have a portable or an onboard RV unit, make sure to read the user’s manual of your generator to find out how much oil your generator takes and buy the right quantity. Additionally, follow the proper procedure to change generator oil.

Can I Use Motor Oil in my Generator?

Yes, you can use motor oil to run your generator given you match the grade of oil with the operating temperature of the generator. During the winter months, use a thin oil grade like 5W-30 and 10W-40 when it is hot.

However, if you find the oil grading system complicated, use a motor oil like Mobil 1 which is best for extreme winters and summers.

Can I Use Synthetic Oil in my Generator?

Yes, you can use synthetic oil in your generator without any problem. Synthetic oil is better for small engines as it protects, improves performance, and increases longevity.

However, synthetic motor oil shouldn’t be used with old generators that have been run with regular oil and have a few years of use under their belt. Moreover, synthetic oil costs more compared to conventional motor oil.

Can I Mix Different Types of Oil?

Yes, you can mix two different types and brands of engine oil with no major issue. Be it synthetic with conventional, 5W-30 with 10W30, or Shell with Castrol.

However, mixing two different types of oil won’t give you the complete benefit of any one oil. Like mixing a synthetic oil with a conventional oil will not give your engine the complete benefit of the synthetic oil.

Moreover, mixing motor oil should not be practiced regularly as different oil types and brands have different oil compositions. This can put off the chemical balance in oil.


The generator oil type you should use mainly depends on the operating temperature of the generator. For winters you need a thinner oil and a thicker oil for summers.

However, make sure to follow the manufacturer’s recommendations and use the motor oil from a reputable brand.