Short answer
SAE J300 is the SAE International standard that defines engine oil viscosity grades. The number before the W is a winter grade set by low-temperature cranking and pumping tests, and the number after it is set by kinematic viscosity at 100 °C plus a minimum high-temperature high-shear (HTHS) viscosity at 150 °C. A multigrade such as 5W-30 meets one winter grade and one non-winter grade at the same time.
Winter grades: the number before the W
Winter grades run from 0W to 25W. Each is defined by two cold tests: a cold-cranking simulator (CCS, ASTM D5293) that reflects how hard the engine is to turn over, and a mini-rotary viscometer (MRV, ASTM D4684) that checks whether the oil can still be pumped to the bearings.
A lower W number is tested at a colder temperature. A 0W oil, for example, must stay under 6,200 cP in the cranking test at −35 °C and under 60,000 cP in the pumping test at −40 °C. The W stands for winter, not weight.
Non-winter grades: the number after the dash
The second number is defined by a kinematic viscosity range at 100 °C and a minimum HTHS viscosity at 150 °C. A 20 grade must fall between 6.9 and just under 9.3 cSt at 100 °C with HTHS of at least 2.6 cP; a 30 grade between 9.3 and just under 12.5 cSt with at least 2.9 cP.
The 40 grade has two HTHS floors: 3.5 cP for 0W-40, 5W-40 and 10W-40, and 3.7 cP for 15W-40, 20W-40, 25W-40 and straight 40. That is one reason diesel 15W-40 oils and European 5W-40 oils are formulated differently even though both are “40.”
The newer low-viscosity grades
As automakers chased fuel economy, SAE added the 16 grade in 2013 and the 8 and 12 grades in its 2015 revision, with HTHS minimums of 2.3, 1.7 and 2.0 cP respectively. Their kinematic ranges overlap on purpose, and the standard tells blenders how to assign a single grade when an oil falls in an overlap.
What J300 does not tell you
J300 describes viscosity only. It says nothing about additive chemistry, deposit control, wear protection or oxidation life; those are covered by service categories like API SQ or CK-4, ILSAC standards and automaker specifications. A correct viscosity grade with the wrong specification can still be the wrong oil.
Frequently asked questions
Is 0W-20 thinner than 5W-20 once the engine is warm?
Both must fall within the same 100 °C range and meet the same HTHS minimum for the 20 grade. The difference is how they behave when cold: the 0W must pass colder cranking and pumping limits.
Does a straight-weight oil like SAE 30 follow J300?
Yes. Monograde oils meet only a non-winter grade (or only a winter grade). They are still common in some small engines, racing and older equipment.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].