Combined efforts by oil companies and engine manufacturers have improved the overall efficiency and performance of two-cycle gasoline engines. One striking trend has been toward the use of leaner oil-fuel mixtures to accomplish lubrication. The use of less oil reduces smoking and air pollution (important in two-cycle automotive engine operation), reduces engine port deposits, preignition, and cost of operation. Experience has shown that the composition of the oil becomes increasingly important as its concentration in the fuel is reduced. Lean mixtures of some conventional oils are not satisfactory, and with all oils some lower concentration is reached where serious problems are encountered, such as lack of lubrication, increased engine wear, and decreased engine cleanliness.
This paper reviews the problems involved in the development of oils specifically for operation as lean mixture lubricants in water-cooled outboard engines, European two-cycle automotive engines, and small aircooled engines. Although the problems vary in each type of engine, data are given on the effect of concentration of lubricants on various performance problems including engine cleanliness, spark plug fouling, preignition, lubrication, rusting, and corrosion.