Oil Grades: Understanding the Secrets of Viscosity

SAE-30, 5W-20 ... oil grades can be confusing. Let's cut through the jumbles of number and letters to understand what they actually represent.

by J. Keeler Johnson
PHOTO: Daniel Johnson

You would think changing the oil in a tractor or ATV would be a straightforward task. And generally speaking, it is. Assuming you have some basic experience working with engines, the trickiest part might be figuring out what type of oil to use.

The challenge stems from the fact not all oils are created equal. It all comes down to oil viscosity—the thickness of oil and how well it flows at various temperatures. Oil viscosity is designated by a series of numbers and letters (“SAE 30,” “15W-40,” etc.). And at first glance, they don’t necessarily mean much.

It’s possible to get along without ever knowing the meaning behind the designations. You can instead just trust equipment user manuals to guide you toward the right oil.

But don’t you really want to get the most out of your machinery? Then it pays to understand oil grades (or “weights”) and what all the numbers mean.

Read more: These are the 7 items you need to perform an oil change.

About Oil Grades

Oil grades are based on the SAE J300 standard, which allows various grades to be directly compared. For starters, you can divide oils into two broad categories—single-grade oils and multi-grade oils.

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The single-grade category further subdivides into winter-grade oils (marked with a “W” and intended for use in cold temperature) and non-winter oils (intended for use in higher temperatures).

Since oils tend to thicken in cold weather, it can be difficult for engines to start unless they’re using winter-grade oil that will resist turning to sludge in sub-freezing weather. The number preceding the “W” in a winter-grade oil measures the minimum temperature the oil in question is suitable for. Lower numbers are superior for colder temperature.

A 0W or 5W oil will flow even if your engine is running in temperatures below 0 degrees F.

Read more: Follow these tips when starting a finicky tractor in cold weather.

Standard Oil Grades

Standard oil grades measure viscosity in a different manner. At the high operating temperatures found in a running engine, oil might thin out too much for ideal performance, particularly if the air temperature is also hot.

Thus, standard oil grades like 30, 40 and 50 measure viscosity at 212 degrees F. Higher numbers indicate thicker oil.

Here’s the problem—the thin oils necessary for starting an engine in cold weather don’t provide as much protection from wear and tear as thicker oils designed for use at engine operating temperatures.

This is where multi-grade oils come into play.

Multi-Grade Oils

Additives allow the creation of multi-grade oils like 5W-30 that combine the best of both worlds. 5W-30 performs like 5W oil when it’s cold, but then shifts and acts like straight 30 when the engine temperature reaches 212 degrees F.

Of course, once you dig into the nitty-gritty details of single-grade oils vs. multi-grade oils, and the pros and cons of the multi-grade additives, and the merits of using thicker oil in old engines to improve oil pressure … it can all get very confusing. Which is why most folks turn to equipment user manuals and purchase whichever oil the manufacturer recommends.

But understanding the basics of oil grades, and what the numbers actually mean, is a useful steppingstone toward a greater understanding of engines in general.

Curious farmers who dig deeper can become capable mechanics in their own right, choosing just the right oil to keep their equipment running in tip-top shape no matter the temperature or age of the engine.

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