Viscosity (oil)

Viscosity describes how thick a fluid is and how much it resists flowing. For engine oil, it decides how well the oil protects parts when cold and when hot. Oil becomes thinner as it heats up, so multigrade oils are formulated to stay within a useful range.

An SAE grade such as 10W-40 has two parts: the number before the W (winter) shows cold-flow behaviour, so a lower number flows more easily at low temperatures; the number after it shows viscosity at operating temperature. Fork oil, brake fluid and gear oil have their own viscosity ratings.

Use the viscosity your owner's manual specifies. For motorcycles with a wet clutch, choose an oil that also meets the JASO MA or MA2 standard, as some car oils contain friction modifiers that cause clutch slip.