What you need to know about oil viscosity.

What you need to know about oil viscosity.

March 26, 2020 1 By Ray Bohacz

Few subjects other than farm equipment “color” loyalty bring more heated discussions (arguments may be a better term) than engine oil brands.

Regardless of your brand preference, the Society of Automotive Engineers (SAE), along with the American Petroleum Institute (API) has defined the quantifiable characteristics of engine lubricants that the industry adheres to, with viscosity being one of them.

The viscosity of a fluid is its resistance to flow. Some think of it as thickness, but that would only be a partial truth. Viscosity defines the force required to move a given layer of fluid past another layer at a given speed and standard separation.

Engine oil is usually a multi-viscosity blend, such as 15W-40. The W does not mean weight but winter. Thus, the oil will flow as if it were #15 when cold and respond as #40 when hot.

Multi-viscosity oils were developed years back to provide the best protection to the engine under all temperature extremes.

The rating does not tell the entire story

If you look at many brands of engine oil, and they all have the same viscosity, a convincing argument could be made that there is no difference. That would be incorrect.

Numerous factors make oil a premium or just an acceptable lubricant. They are based on the crude employed, the refining process, additives used, along with a host of other things. For this discussion, we will only be concerned with the viscosity.

Let’s say you were harvesting corn, and you took two points from the field and recorded the yield from the monitor. The opening of the field produced 200 bushels/acre, and in the middle of the field, 305 bushels/acre.

A true statement would be the ground around the entry to the field yielded 200 bushels/acre and the middle 305 bushels/acre. That is the field’s performance for those two points but tells you nothing about the yield in the other areas. That is what the rating on the oil container is saying.

The rest of the story

The viscosity rating is the oil’s performance at the assigned temperatures for cold and hot. The cold temperature test is at 0 F degrees, and the hot rating is at 212 F degrees.

The oil (15W-40 as an example) acts as if it were 15 weight at 0 F degrees and as 40 weight at 212 F degrees.

We only know its performance at those two temperatures. To indeed qualify an oil, a viscometric index chart is required, and no one publishes that.

Simply put, most inexpensive oils skew significantly in their viscosity above and below the test temperatures, and that is where your engine will experience excessive wear.

Use high-quality name brand oil, and it will have more stable and linear performance up and down the temperature range.

The debate goes on

Over the years, I have taken many engines apart and have found the entire gamut of wear and cleanliness.

When the name brand engine oils were used and changed with some regularity, the engine told you so inside. It was clean; the bearings looked good along with the cylinder wall finish and crosshatch.

Cheap oil announced its presence loudly with excessive deposits, gunk and odor, extreme wear of all parts, and poor cylinder wall condition.

Use the top-of-the-line engine oil of the brand you like with the proper specifications, and all will be well.

With newer equipment, you must use engine oil that meets that manufacturer’s specifications.

Things are not simple anymore; internal design features, materials, and machining often demand dedicated additive packages in the engine oil.

This may be burdensome when you have many different engines on the farm of varied makes and years.

Resist the temptation to use one oil type for every engine if there are specific needs.

It is less burdensome to stock the proper lubricants for your equipment than to spend your time and money on a premature rebuild.