text: Base Number (BN) is a measurement of basicity that is expressed in terms of the number of milligrams of potassium hydroxide per gram of oil sample (mg KOH/g). BN is an important measurement in petroleum products, and the value varies depending on its application. BN generally ranges from 6–8 mg KOH/g in modern lubricants, 7–10 mg KOH/g for general internal combustion engine use and 10–15 mg KOH/g for diesel engine operations.  BN is typically higher for marine grade lubricants, approximately 15-80 mg KOH/g, as the higher BN values are designed to increase the operating period under harsh operating conditions, before the lubricant requires replacement.