Generator oil

Generator oil is an important part of keeping your generator running smoothly.

Different generators require different types of oil. It’s important to use the right type of oil for your generator and follow the manufacturer’s recommendations for viscosity, operating temperature, and engine type.

Follow our detailed guide, and you’ll never have to worry about your generator not performing at its best. Plus, you’ll be doing your part to keep it running smoothly for years.

Read through our detailed guide on generator oil!

Role of the generator Engine Oil

A large portion of the heat generated in the combustion cycle is carried away by the exhaust gases or radiated by the metallic components of the engines. However, still enough heat remains to affect the long-term functionality of the engine parts. The two systems responsible for getting rid of this remaining portion of heat are the lubrication system and the cooling system.

Engine oils are tasked with removing this heat in conjunction with the airflow system and reducing the friction between engine components which is another major source of heat generation.

In addition, it neutralizes acids, cleans out sludge, and inhibits oxidation and corrosion.

Reducing Friction with Engine Oil

The most critical quality impacting your oil’s performance is viscosity, which describes the ability of any liquid to resist motion. The moving parts like pistons will try to push the oil away. Oil must have the capacity to resist this displacing force and maintain a continuous film of uniform thickness to keep the two metallic parts from touching each other. Viscosity is instrumental in allowing any oil to cling to different types of surfaces.

On the other side, if the oil is too viscous, it will create more construction to the moving parts and reduce the efficiency of the engine.

An increase in viscosity reduces the ability of any oil to flow freely at any given temperature. A more viscous oil will reach its optimal temperature in a comparatively longer duration. In addition, the engine oil has a high viscosity at lower temperatures that decreases with an increase in temperature. The low viscosity oil is thin and flows easily.

The viscosity index (VI) guides a user about the temperature’s impact on the viscosity change. As a general rule, the viscosity of the oil with a low VI value will be impacted more by the temperature change.

As explained above, the viscosity must be high enough to create a film of sufficient thickness that prevents metal-to-metal contact under operating temperatures, even under load. On the other hand, it must not be that high to cause a drag on the moving parts under the operating conditions.

Effect of Temperature on Oil Life

Oil life is halved with every 10° rise in the base temperature of the oil.

Oil Circulation

A dipper or a slinger located in the crankcase of the engine distributes oil in most four-stroke engines by their splashing movement. The oil is stored in a reservoir in the crankcase. Oils seals do the job of containing the oil in the crankcase for 4-stroke engines.

In a horizontal crankshaft engine, a dipper is fitted to the connecting rod, which picks up the oil from the reservoir and splashes it over all the bearing surfaces. A slinger with spinning gear and paddles cast onto a plastic gear body does the same job in a vertical crankshaft engine.

In many OHV-type engines, the manufacturer might include an oil pump when the valve gear is remote or your engine has an oil filter. Some designs even carry passages drilled onto the crankshaft to carry the oil to the crankpin and main bearings.

Selecting the best oil for your engine

with this brief introduction to the role of engine oils and their distribution in small generators, let us review in-depth how to select the best oils for your portable generator.

Look Into Your User’s Guide

Start with an in-depth review of the relevant sections of the owner’s manual that came along with your generator. Most manufacturers provide details about the quantity and type of oil to be used along with the frequency of replacement in the maintenance section.

Generator Engine Oil Types

Engine oils are generally classified into three types – Mineral oil, semi-synthetic oil, and synthetic motor oil. Let us briefly discuss them.

The Mineral Oil

The main features of the crudest Mineral Oil are:

  1. The crudest oil of the lot. It is a refined petroleum oil treated to perform under a wide range of temperatures.
  2. Cheapest out of the three.
  3. They offer no protection against friction-induced heat.
  4. Ineffective at lower temperatures.
  5. Susceptible to breaking down at high temperatures.
  6. Require more frequent replacement.

The Semi-Synthetic Oil

The semisynthetic oils provide the best of both worlds, as they are affordable with a real boost in performance compared to the mineral oils. They are also known as synthetic blend oil. Semi-synthetic oils are manufactured by mixing a small quantity of synthetic oil with mineral oil.

  1. The addition of synthetic oil improves its viscosity and wear resistance under stress and high-temperature conditions.
  2. They exhibit better performance under low operating temperatures compared to mineral oil.
  3. Their main disadvantage is their inability to offer a superior level of protection.

The Full Synthetic Oil

This is the best, high-quality oil that provides better engine protection and high efficiency. Full synthetic motor oil molecules have a great degree of consistency in size and shape and provide a much higher degree of lubrication. Its performance is optimal both under hot and cold temperatures, even under extraordinary stress.

The Oil Viscosity

While talking about engine oils, you will frequently encounter the term SAE, which stands for the “Society of Automotive Engineers.” SAE has developed a numerical code-based grading system that uses viscosity as its basis. SAE 30 is the most commonly recommended oil for small 4-stroke engines.

In the past, the focus was to start with thick oil to ensure that after thinning due to high temperatures, it is still able to provide the desired film thickness. However, this leads to starting problems in winter conditions, when the increased viscosity of the oil makes it difficult to crank the engine.

The solution is to alter some of the oils to make them less viscous during winter. These multi-viscosity oils are designated by ratings such as SAE 10W-30. In this nomenclature, 10W, the first number, denotes the winter viscosity with a lower designation (less viscous) of 10 compared to 30, the second number, during warmer temperatures.

Multi-viscosity oil may pose some problems if you continuously operate your equipment at higher temperatures, like premature carbon buildup and loss of engine power. You must contact the supplier to obtain specific recommendations.

American Petroleum Institute (API) gives a service rating and certifies that the engine oil meets the OEM standards. Look for a service rating of SJ, SL, SM, and SN/SN+. Other categories are now obsolete and may cause harm to your engine.

Also, note that SAE 5030 is not the same as SAE 5W-30.

Operating Temperature

Generally, the owner’s manual carries a chart, as shown below, which specifies the grade of oil recommended for the expected temperature range. The attached chart recommends the use of

  • SAE 30 for temperatures of 40°F and above.
  • SAE 10W-30 for temperatures between 10°F and 40°F.
  • SAE 5W-30 for temperatures lower than 10°F.

As you can see, extremely cold conditions will require better viscosity oil, which implies a further reduction in viscosity as the temperature decreases.

Type of Engines

So far, what we have been discussing applies mainly to the 4-stroke engines, as most generators in the market are of this type. Let us now briefly touch upon the requirements of two-stroke engines. Please note that the engine oils are different for both these types of engines.

For two-stroke engines, the engine oil is divided into two categories: air-cooled two-cycle and water-cooled engines. Air-cooled engines have higher temperatures than water-cooled ones. However, synthetic oils perform equally well in both types.

Two-cycle engine oils may contain a fuel stabilizer as the oil is directly added to the fuel in a recommended ratio. You can also purchase a pre-mixed solution. Marine-grade 2-cycle engine oil is used with outboards and other water-cooled power equipment.

Reputed Oil Brands

It is always recommended to go with a reputed and well-known brand despite little higher prices, as far as the engine oil is concerned.

Some of the reliable brands are

  • Generac Motor Oil
  • Shell Rotella Oil
  • Honda
  • Onan
  • Briggs & Stratton generators

How to Change Oil in a Generator 

The entire process of changing the oil in your generator is very straightforward. Ideally, follow the instructions provided in the owner’s manual. If for some reason, your manual does not carry these instructions, you can follow the general steps presented below:

  1. Begin by disconnecting the battery cable as a safety precaution.
  2. Disconnect the spark plug lead and secure it away from the spark plug.
  3. Position a newspaper and an oil pan below the drain plug
  4. Open the drain plug with a socket wrench slowly, allowing the oil to drain into the oil pan.
  5. Close the drain plug after the complete oil has drained out.
  6. Open the filler tube and add the correct amount of engine oil by weight using a funnel until it reaches the tube’s thread line. Measure the oil level using a dipstick.
  7. Run the engine idle for a few minutes and observe any leaks.

This helpful video demonstrates the process.

FAQs about the Generator Oil

Let us now try to answer some common questions that we frequently encounter.

Can I Use Synthetic Oil in a Generator?

As explained above, synthetic oil has better properties than semi-synthetic and mineral oils. But, they are more expensive than others. Hence, there is no issue in using them as long as you are meeting the requirements stated in your owner’s manual and following the recommended oil change frequency.

What Could Happen When I use too Little or too Much Oil?

As explained at the beginning of the article, the engine oil carries out the function of lubrication and cooling the engine. Your engine will suffer internal damage and deteriorate faster. To protect against these conditions, most engines carry lower oil levels and low oil pressure protection.

Over-filled oil will try to find a path such as vents and air filters to escape. It may find its way to the cylinder causing heavy smoke. A thicker layer of the oil will increase the friction for the moving parts and decrease the efficiency of the engine. Further, the oil pump may not be able to develop the required pressure, affecting the lubrication.

Can I Use a Different Brand of Oil in my Generator?

If the generator manufacturer has specified the brand, viscosity, and other details, it is always better to stick to these recommendations. If no specific details are provided, you can go with any reputed brand selecting the viscosity based on the temperatures prevalent in your area.

What’s the Difference Between Single-Grade and Multigrade Rating?

This topic has been dealt with in detail in our article in the sections on “the oil viscosity” and “operating temperature.” To summarize, SAE 30 is a single-grade oil, while SAE 10W-30 is a multigrade oil.

How Often Should I Change the Oil in my Generator?

As with other things, the first step is to check your owner’s manual for any specific recommendations by the manufacturer. If no specific guidelines are provided, you may carry out the very first replacement after 25 to 30 hours of running and every hundred hours thereafter.

The use of synthetic oil allows you to double this time.


In conclusion, using the correct type of oil in your generator is important. The brand of oil you choose will depend on various factors, such as the make and model of your generator, the climate you live in, and the engine type. By following our simple guide, you can be sure to select the right oil for your needs. Have any questions? Let me know in the comments section below!

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