In this study , an important overview is given about different lubricating oil available in the market and their properties. Special attention is given to the property "viscosity".Lubrication is an important phenomenon in engineering sector mostly related to mechanical engineers. Almost all mechanical devices require lubrication to their moving parts. Without proper lubrication, life of the machine reduces. Varieties of lubricants are widely used in automotive engines. The study found some differences between the same category oil of different manufacturers.This book will help readers for selection of proper lubricating oil. At last some suggestions have given for the future research and practical application of the lubricating oil.