Knowing how engine oil is rated and how ratings apply to your tractors and combines can help you make a better decision at purchase. Here, major lubricant and machinery company representatives tell us what the classifications mean and how they apply to machinery.

Performance categories. The American Petroleum Institute performance classifications (known as API) are a uniform set of standards to assure acceptable long-term engine performance. "These classifications keep changing as engine technology improves and as engines keep generating more power without getting much bigger in physical size," says Charles Glomski, Case IH materials engineering.

For gas-powered engines, API SJ [or the International Lubricant Standardization and Approval Committee's ILSAC GF2] are the most recent ratings. "For diesel-powered equipment, oil performance categories begin with the letter C [compression-ignition]," Glomski says. "Look for API classification CH-4 (preferred) or CG-4."

According to API, CG-4 may also be used in engines that require CD, CE and CF-4 oils. "CH-4 is the latest and highest performance category for diesel engines," says Harold Tucker, lubricants technical director, Phillips 66. "It's best used for diesel equipment that needs multi-viscosity oil; also, CF-4 is for some diesel engines that require straight grade oil."

The API states that CH-4 is superior to oils meeting CF-4 and CG-4 and may be used in place of those. Tucker emphasizes that operators should "look for the 'donut' symbol on the package; then make sure it contains the most recent rating."

Thick or thin. Viscosity classifications are established by the Society of Automotive Engineers (SAE). There are 12 viscosity grades suitable for engine oils. Because lower viscosity weight is a thinner oil, it can handle the stress of cold weather.

"Base oil and performance package constitute the chief ingredient of a lubricant, followed by a viscosity improver and pour-point depressant," according to Jeff Samet, product manager, lubricants, AGCO.

Glomski says the top three major ingredients and/or property values to look for in engine oil are total base number (TBN) with a minimum of 11 (but a retention of 9 or 10 is sufficient, according to Samet); zinc with a minimum of 0.16%; and sulfated ash with a minimum of 1.4%.