On 5w30 oil, what does the "5" indicate?

The "5" in 5W30 oil indicates the viscosity rating of the oil at low temperatures, specifically how it behaves in cold weather conditions. The "W" stands for winter, which signifies that the oil is formulated for cold conditions. Thus, the "5" means that the oil will flow properly at temperatures down to -18°C (0°F), allowing for easier starting of an engine in cold climates.

In conjunction with the "30," which reflects the oil's viscosity at higher operating temperatures (typically around 100°C), this number designates that 5W30 oil is a multi-viscosity oil, engineered to perform consistently across a range of temperatures. This characteristic is particularly beneficial for engines that need reliable oil flow and protection in varying environmental conditions, hence making 5W30 helpful for both cold starts and high-temperature engine performance.

While single viscosity oils maintain a constant viscosity, multi-viscosity oils adapt to changing temperatures, thus providing better performance and protection for engines during all seasons.