This paper discusses lubricating oil field tests conducted in two-cycle and four-cycle engines fueled with natural gas. The main objective was achieved; i.e., high performance lubricants were developed with low ash-forming properties. In the four-cycle engine program, it was found that (a) good oil oxidation stability was a primary requirement, (b) increasing either detergent or oxidation inhibitor level reduced piston ring belt deposits, and (c) a compounded paraffinic oil was markedly superior to an equally compounded naphthenic oil. In the two-cycle engines, (a) increased detergent concentration reduced port deposits, and (b) piston head deposits correlated with the oils' ash content.Contributed by the ASLE Technical Committee on Engine Lubrication and presented at the Annual Meeting of the American Society of Lubrication Engineers held in Philadelphia, Pennsylvania, April 1961.