Viscosity measures a fluid’s resistance to flow, reflecting its internal friction. High viscosity means the oil is “thicker” or “heavier,” while low viscosity means it’s “thinner” or “lighter.” For example, honey is thicker (higher viscosity) than water.
Viscosity inversely relates to fluidity and changes with temperature. Hotter oil becomes thinner, reducing engine protection, while colder oil thickens, making it harder to pump and protecting less at start-up.
Oil viscosity must balance: thick enough to lubricate but thin enough to flow easily through the engine. Contamination also affects viscosity—soot, dirt, and sludge increase it, while fuel dilution decreases it, both harmful to the engine.