Severe temperature load of engine oils effects their relatively rapid thermo-oxidation. Some types of combustion engines cause, a particularly high permanent temperature load of engine oil. Gas combustion engines of cogeneration units of biogas plants are also operated in a similar way. These combustion engines operate for long periods at full load and constant speed. Biogas produced by fermentation of waste organic materials is used as a fuel. Intensity of degradation of engine oil is caused most often by high and long-term temperature load, total number of wear particles and contamination of combustion products. Content of combustion products depend on the individual content of crude biogas. The aim of the paper is to examine partial effect of long-term temperature load and thermo-oxidative stability of selected types of engine oils for gas engines used in cogeneration units. Mineral oil, hydrocracked oil and full synthetic oil was tested. All of selected oils are recommended for use in gas engines. Oil lubricity was tested on Reichert tester device during standard conditions. As results show, for some types of engine oil a long-term temperature load had positive effect and for some types of engine oil a long-term temperature load had adverse effect on lubricity.