Several MIL-L-2104C lubricants were evaluated in a small air-cooled rotary combustion engine, using leaded and unleaded gasoline. A 50 h endurance test was developed which discriminates lubricant quality on the basis of combustion chamber, spark plug, and side plate deposits. Additionally, use of unleaded fuel resulted in an eventual decrease in engine performance at approximately the 40 h mark. This is attributed to the character of apex and side seal groove deposits which caused seal spring binding and consequent blow-by. The presence of sulfur in each fuel may also have an attenuative effect on such binding.