Motor Oil Buying Guide

Motor oil from bottle

Selecting the right type and grade of motor oil isn't just about keeping things running; it directly influences engine health, performance, and long-term reliability. An incorrect oil choice can lead to premature wear, decreased fuel efficiency, or even engine failure. This guide provides information about motor oil types, grades, and the various international standards.

What are the Common Types of Motor Oil?

Full Synthetic: Manufactured using the highest-grade base oils, fully synthetic motor oil provides excellent protection against wear and tear, heat, and oxidation. Synthetic oils are better able to withstand extreme temperatures and are often suited for the demands of today’s engines. It can also handle extended drain intervals, saving you time and money in the long run. Synthetic motor oils can normally be used in place of conventional motor oils. However, this performance comes at a price – synthetic oil typically costs more than conventional alternatives.

Synthetic Blend (Semi-Synthetic): A synthetic blend is a mix of conventional and synthetic oils. By blending synthetic and conventional base oils you get a significant performance boost over conventional oil. Expect improved resistance to breakdown, smoother cold starts, and enhanced engine protection. Synthetic blend provides some of the properties of full synthetic with prices that are more comparable to conventional oils.

Conventional: Conventional motor oil is refined from base or crude oil. It is available in a range of viscosity grades and is an affordable option. It is best used on light-duty vehicles with simple engine designs. Conventional oil may require more frequent oil changes than synthetic or semi-synthetic options.

High-Mileage: Vehicle engines with 75,000 miles or more on their odometer could benefit in using a high mileage oil. This type of oil has specific additives like seal conditioners and viscosity modifiers. These additives help to reduce leakage by inflating o-rings and gaskets.

Motor Oil Grades Explained

Oil flowing through pistons

The Society of Automotive Engineers (SAE) established the motor oil viscosity grade system. This system categorizes motor oils based on their viscosity, which is a measure of the oil’s resistance to flow. When determining the correct viscosity grade for your vehicle it’s always best to consult your owner's manual. To explain the viscosity grade of oil we will use 5W-30 multi-grade engine oil as the example.

The "W": Stands for "winter." The number before the "W" (e.g., 5W) indicates the oil's winter viscosity grade. The lower the number, the better the oil flows at cold temperatures, aiding in engine starts.

High-Temperature Viscosity: The second number (e.g., 30) indicates oil thickness at normal engine operating temperature. Higher numbers mean thicker oil, crucial for maintaining lubrication between components.

Choosing the Right Grade for Your Vehicle

Manufacturer Recommendations: Your vehicle’s owner's manual is the best source for determining the correct grade of oil and oil filter for your engine. Very cold climates may benefit from lower "W" rated oils, while hot regions may need a higher viscosity at operating temperature.

Oil Filter Selection: Oil change intervals have increased on some newer vehicles. Make sure the oil filter installed on your vehicle has the capacity to last until the next oil change. Select a high-quality oil filter that meets or exceeds OEM specifications.

Engine Oil Standards

The Automotive Petroleum Institute (API) provides a series of standards and certification programs specifically for motor oil. To ensure that you are buying quality motor oil for gasoline and diesel powered vehicles it’s important to understand the certification marks and standards that are shown or listed on a bottle of motor oil.

API Service Category – API places motor oils into several service categories to indicate their suitability for different types of engines and operating conditions. These categories are divided into two main groups: "S" (Service) categories for gasoline engines and "C" (Commercial) categories for diesel engines. The latest category for gasoline engines is “SP” and “CK-4” for diesel engines.

API Service Symbol or Donut – This round symbol will be shown on the motor oil bottle label and includes the API service category and viscosity grade (e.g., 5W-30). It may also say “Resource Conserving.” This applies to oils intended for gasoline-engine cars, and light trucks that meet performance standards that are aimed at improving fuel economy and reducing emissions.

The International Standardization and Approval Committee (ILSAC) also sets standards for motor oils used in gasoline engines. It’s a collaboration between American and Japanese automobile manufacturers. These standards are normally updated every few years to keep up with advancements in automotive technology and environmental regulations.

ILSAC Gasoline Engine Oil Rating – The ILSAC rating for gas powered vehicles can be found in your owner’s manual and it will also be listed on the label of a motor oil bottle. Current rating standards are “GF-6A” and “GF-6B.” Although both standards address Low-Speed Pre-Ignition (LSPI), motor oils assigned with “GF-6B are formulated to mitigate LSPI better than “GF-6A” oils. “GF-6B” also covers viscosity grade 0W-16.

Additional Considerations for Oil Selection

Driving Style: Aggressive driving or frequent towing puts extra strain on oil, potentially warranting synthetic oil and potentially shorter change intervals.

Performance/Modified Engines: These often have unique lubrication and filtration needs. Consult with the engine builder or part manufacturer for guidance.

Addressing Oil Consumption Issues: High-mileage oils or slightly thicker grades can sometimes help reduce consumption, though underlying mechanical problems may need to be addressed.


Motor oil is deceptively complex. By understanding the types, grades, standards, and importance of oil filters, you can make informed recommendations that safeguard engines and optimize performance.