What the numbers on a motor oil bottle really mean

A motor oil grade such as 5W-30 is shorthand for how thick the oil is at different temperatures. It tells you how the oil behaves on a cold start and once the engine is hot, and it is only one part of the specification that matters.

Two numbers, two jobs

The grade comes from SAE J300, the standard that classifies engine oil by how freely it flows and the one engine makers use when they choose oil for a model. It defines two series: grades carrying the letter W and grades that do not. A grade such as 5W-30 belongs to both, so it must meet a cold requirement from the W series and a hot requirement from the other. The sae.org standard sets those limits.

What the letter W changes

The W marks a grade with limits for low-temperature cranking and pumping, the two things a cold engine needs the oil to do. The number in front of it describes that cold behaviour: a 0W has to flow at lower temperatures than a 5W, and a 5W lower than a 10W. A lower number means the oil stays pumpable on a cold morning, which is why the cold grades carry the W.

The number after the W is the hot grade

The number after the W is the oil's viscosity grade at 100 degrees C, roughly normal running temperature. A 30 is thicker than a 20 in that range, and a 40 thicker than a 30. That is where the protection and efficiency trade sits: a thicker oil carries more load, while a thinner one takes less energy to pump. Multigrade oils such as 5W-30 are defined by both the cold limits and the hot grade at once.

The grade is only half the label

Two oils can share a viscosity grade and still differ in performance. The API service category on the container is the other half. For petrol engines the latest category includes the properties of the ones before it, so an oil meeting API SP also covers an engine whose manual asks for API SN. ILSAC's current passenger car standard, GF-6, was introduced in May 2020. The api.org marks explain what each label means.

Why the manual's grade helps fuel use

Using a thicker grade than the engine was designed for costs fuel. The U.S. Department of Energy puts the penalty at 1% to 2% when an engine specified for 5W-30 is filled with 10W-30, and 1% to 1.5% for 5W-30 in an engine designed for 5W-20. Oils carrying the API Energy Conserving mark hold friction-reducing additives, as the figures on fueleconomy.gov show.

Bottom line: the code on the bottle is a cold number and a hot number, and the owner's manual decides which pair belongs in the engine. Match both the grade and the service category the manual lists rather than reaching for the thickest oil on the shelf.