How to Select Lubricating Oil for Farm Engines

Engine lubricating oil must perform several functions. It should:

  • Reduce friction and wear between moving surfaces
  • Remove heat caused by friction
  • Provide a seal against escaping gases
  • Keep the engine clean by holding carbon and sludge-forming material in suspension, so they will be removed by the oil filter or when the oil is changed
  • Provide protection against rusting and attacks by acids.

Consider these factors when you select engine oil:

  • Oil types (API service classifications)
  • Oil viscosity
  • Operating conditions.

API service classification. This system, developed by the American Petroleum Institute, provides guidelines for the selection of crankcase oils suitable for various service conditions. It classifies general ranges of engine service needs based upon:

  • Engine design and construction
  • Lubricating oil
  • Operating conditions
  • Maintenance practices
  • Fuel characteristics.

The API engine service classification system presently includes 13 classes of service — seven for automotive (spark ignition) engines and six for diesel (compression ignition) engines. Five of the automotive and two of the diesel service categories are obsolete. Only currently recommended categories are listed in this guide.


Engine lubrication oils have chemical compounds or additives added to them for improved performance. Some of these additives are:

  • Oxidation inhibitors
  • Detergent dispersants
  • Corrosion inhibitors
  • Rust inhibitors
  • Anti-foam agents
  • Anti-wear agents
  • Viscosity index improvers
  • Pour point depressants.

Every good oil does not necessarily include each of these additives.

Packaged additives. Mixing additives with modern engine oil is not recommended. There is the possibility that their use could upset the chemical balance of the engine oil and its original additive system or shorten the engine oil's serviceable life, and may even prove detrimental to the engine.

Oil viscosity

Viscosity is the measure of the resistance to flow. It is the body or thickness of the oil. Viscosity is not a measure of oil quality.

Poor-quality oil can have the same viscosity classification as a good oil. Seven viscosity categories are defined by the Society of Automotive Engineers (SAE). These are SAE 5W, SAE 10W, SAE 20, SAE 20W, SAE 30, SAE 40 and SAE 50.

The "W" (for winter) following a viscosity number indicates that an oil is suitable for cold temperature and must have the indicated viscosity at 0 degrees F. The SAE categories that do not include the "W" are suitable for use at high temperatures and must have the specified viscosity at 212 degrees F.

A multiviscosity oil meets an SAE viscosity requirement at both 0 degrees F (-18 degrees C) and 212 degrees F (100 degrees C). It does not thin out as much when heated or thicken as much when cooled as does a single-viscosity oil.

For example, SAE 5W-30 and 10W-30 oils meet cold ranking requirements of SAE 5W at 0 degrees F and high temperature viscosity requirements of SAE 30 at 212 degrees F.

Thus, a multiviscosity oil stretches the usable temperature range. It provides easier cold-weather starting, more efficient lubrication, reduced engine wear, better fuel economy and adequate protection against excessive oil thinning at operating temperatures.

Source of contamination

Combustion of hydrocarbon fuels such as gasoline and diesel fuel forms byproducts that cause corrosion and engine deposits. For example, each gallon of fuel burned causes about 1 gallon of water to be formed. Most of the water forms as vapor and goes out the exhaust. However, a small amount condenses on the cylinder wall (especially when the engine is cold) and eventually is trapped in the oil reservoir.

Carbon (or soot) formed by incomplete combustion of fuel is also picked up by the oil and carried into the oil reservoir. In combination with water, the carbon forms sludge which, if allowed to accumulate, may restrict oil passageways and cause insufficient oil flow to engine parts.


Information in this guide sheet should not replace the operator's manual recommendations. Always follow all engine oil specifications and use the manuals furnished by the engine manufacturer.