The paper describes a new and original method of utilising known design features of 4‐stroke and 2‐stroke trunk piston diesel engines for the purpose of enabling accurate selection of the correct type of lubricating oil to be made, in terms of performance level. Assessment is made on the basis of matching test engine parameters with those of the engine under examination. Slow speed crosshead 2‐ and 4‐stroke diesel engines with separate cylinder lubrication are not included. The important parameters of piston speed, b.m.e.p, heat input factor, and fuel sulphur content are used and graphs are provided which permit rapid selection of the type of lubricant most suited to meet the engine requirements. Lubricants are graded in terms of their ability to meet well known specifications which define performance standards by engine test techniques.