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What's the Difference in Car Oils and What's the Best Oil to Put in Your Car?

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Dan has been an auto mechanic, truck driver, engineer, and many other things.

What Is Oil and Why Is It in My Engine?

Oil is the substance that lubricates the internal moving components of your vehicle's engine. There are many different types of oils on the market suitable for car engines but there are some basics that you should know when purchasing an oil.

Viscosity

This is simply how readily the oil flows. A higher-viscosity oil will withstand more heat than a lower-viscosity oil. However, at colder temperatures, it can actually become so thick that your starter cannot turn over the engine—and worse yet, it won't flow to all the moving parts it should. A viscosity that is too low won't maintain a lubricating film on engine parts, allowing them to make metal-to-metal contact. Either extreme is very bad!

Single-Grade Versus Multi-Grade

Single-grade engine oils (such as SAE 30) used to be the standard for engines operating at high temperatures (such as air-cooled engines). Since the advent of the American Petroleum Institute (API) rating system, the multi-grade oils that proliferate on shelves nowadays are suitable for nearly all applications. I did have an older gentleman tell me today that his '30ish Ford won't build oil pressure with multi-grade oil—but then again, I'd be surprised if his '30ish Ford builds oil pressure at all.

Coincidentally, SAE stands for Society of Automotive Engineers.

Selecting Your Grade

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So, which oil you should choose will mostly depend on what you're driving. If your car is not an antique or air-cooled, the safest bet is to use a multi-grade oil. The first number on a multi-grade oil indicates the "cold" viscosity of the lubricant, and the second number the "hot" viscosity. For example, a SAE 10W-40 motor oil would have a viscosity rating of 10 at 0-degrees Fahrenheit (the W indicates "Winter") and a 40 rating at its maximum recommended temperature (not engine operating temperature—ambient air temperature).

The following chart is a good place to start. Determine the temperatures you'll be driving in and select an oil accordingly.

The Breakdown

Multi-Grade Oil Viscosity Chart

Multi-Grade Oil Viscosity Chart

Special Considerations: Driving Conditions, the Age of Your Car, Winter Weather

Also, consider your driving habits and the condition of your car. If you drive hard—keeping the engine near redline for extended periods, or you do a lot of high RPM "freeway" driving—then you may want to select a higher-viscosity oil for your car. Also, if your engine is high mileage, a high-viscosity oil will keep it a little quieter and may not slip through the rings to be burned (blue smoke).

If your car is newer, you will most likely want a lower-viscosity oil. This is true of most newer high-performance cars also—they too have tighter bearing tolerances and recommend a low-viscosity oil (it doesn't rob as much power from the engine).

Overall, If you expect very cold weather ahead, definitely choose an oil with a lower "cold" viscosity. If you live in a warmer region, a higher winter rating (the W rating) will help keep your cam and valves from clattering when you first start your car.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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