What Is Grease, and How Is It Different from Oil? (NLGI Grades Explained)

Grease is often described as "thick oil," but that's not quite right. Grease is oil (the base fluid doing the actual lubricating) combined with a thickener — commonly a metal soap like lithium, calcium, or lithium complex — that holds the oil in a semi-solid structure. Under mechanical pressure (like inside a moving bearing), the structure releases small amounts of oil to lubricate, then re-forms once the pressure is gone.

Why use grease instead of oil at all

Grease stays where you put it. A bearing, a chassis pivot, a wheel hub — anywhere a constant flow of liquid oil would simply drip or leak away — is exactly where grease belongs. It also acts as a physical seal against dirt, water, and contaminants entering the bearing, which plain oil doesn't do nearly as well.

The NLGI scale — grease's version of a viscosity grade

The National Lubricating Grease Institute (NLGI) grades grease by consistency — how firm or soft it is — on a scale from 000 (nearly fluid) to 6 (nearly solid rubber). The scale is determined by how far a standardized cone sinks into the grease under its own weight in five seconds (a "cone penetration" test).

• NLGI 1 — softer, used in centralized/automatic lubrication systems and low-temperature applications.
• NLGI 2 — the most common general-purpose grade, used in a wide range of automotive and industrial bearings. If you're not sure what grade a spec calls for, NLGI 2 is the safest default to ask about.
• NLGI 3 — firmer, used in higher-temperature or higher-load applications, or where a firmer seal against contamination is needed.

Thickener type matters too

Two greases with the same NLGI 2 rating can behave very differently depending on the thickener: lithium and lithium-complex greases are the most common general-purpose choice, with good water resistance and a wide temperature range; calcium-based greases (like calcium AP-3, a common entry product for price-sensitive markets) offer good water resistance at a lower cost but a narrower temperature range.

Choosing the right one

Match NLGI grade and thickener type to the actual application — chassis, wheel bearing, industrial machinery — rather than assuming "grease is grease." Our capabilities page and country-specific recommended-products lists are built around this distinction.