The oil lubrication system is a critical part of the helicopter main gearbox (MGB) and this is evident in the many accidents and incidents over the last 30 years. On a category A rotorcraft, a regulatory requirement mandates the MGB to sustain operation for at least 30 minutes following the loss of the primary oil lubrication pressure. The aim of this study was to undertake a comparative investigation into the performance of mist lubrication, using commercially available thioether (MCS-293™), on a category A helicopter MGB under loss of oil conditions. Experimental observations highlighted that the high-speed input module of the MGB attained the highest temperature and was a limiting factor to continued gearbox operation under loss of oil conditions. Results showed that by routing thioether mist through existing galleries within the MGB a lower rate of temperature increase was achieved, in comparison with a dry-run condition.