SAE J300 defines mono- and multigrade engine oils; for example,  cars may need SAE 0W-20 multigrade engine oils while lawnmowers only need SAE 30 monograde oils.
SAE J300 is a standard that defines the viscometric properties of mono- and multigrade engine oils, maintained by SAE International. To illustrate, a lawnmower may require SAE 30 monograde engine oil that must meet the SAE 30 requirements. In comparison, a contemporary passenger car engine may require SAE 0W-20 multigrade engine oil that must meet both the SAE 0W and SAE 20 requirements. 
