<p>Hydrogen and ammonia have been considered as prominent fossil-free energy source candidates. While their combustion characteristics and emission profiles are well-documented, the implications for engine lubrication systems remain underexplored. This study aims to bridge this knowledge gap by investigating the effect of gaseous green fuels, specifically hydrogen and ammonia, and their influence on lubricants and the tribological performance of the lubricants. Applying a rapid and cost-effective lab-scale ageing process, the ageing effects of gas and gas admixtures were simulated and differentiated in a controlled environment. Detailed chemical, physical, and tribological analyses provide valuable insights into the different degradation outcomes of the lubricants with different exposing gases. The results revealed degradation of lubricant performance after ageing with the gases, due to changes in the lubricant chemistry and, in some cases, viscosity at 100 °C. In all cases, lubricant ageing introduced increase in friction coefficient between steel surfaces and impairment of the lubricant load-carrying capacity. The results obtained from this work will benefit the appropriate selection of lubricant alternatives for future engines utilizing green fuels and facilitate the development of more effective and durable lubrication solutions to support the broader adoption of hydrogen and ammonia as sustainable energy sources in internal combustion engines.</p>