SRV Oscillatory sliding experiments -At 20°C in pure oscillatory sliding testing, it was shown that samples with lead naphthenate used as an anti-wear additive outperformed neat MAC samples in all experiments. Under vacuum conditions, the samples with lead naphthenate offered twice the wear protection with 440C performing the best. -At 75°C in pure oscillatory sliding testing on 52100 steel, samples with lead naphthenate in an atmospheric environment performed worse as the concentration of lead increased. However, when in a vacuum environment, as the concentration increased the wear rate decreased. -At 75°C in pure oscillatory sliding testing on 440C, samples with lead offered up to twice the wear protection over the neat samples in a vacuum environment. -Lead Naphthenate had better anti-wear performance at lower temperatures and increasing the amount of lead directly reduced the wear rate. At higher temperatures the wear rate was almost double for the highest lead loading across all tests. MTM mixed rolling/sliding experiments -At 50°C in the counter-rotation wear test, lead naphthenate offered three times the wear protection when in a 3% concentration and eleven times the protection for 5% lead compared to the neat MAC fluid. -At 150°C in the counter-rotation wear test, lead naphthenate offered five times the wear protection when in a 3% concentration and twenty times the protection for 5% lead compared to the neat MAC fluid. -There appears to be is a transition point between 3% and 5% lead naphthenate where the available lead can react with both the surface and worn metal to create a strong lead anti-wear tribofilm. This tribofilm that is created is between 2-4 µm thick and while it will cause an overall increase in friction at the surface, the wear of the contact is greatly reduced. -Lead Naphthenate had better anti-wear performance at high temperatures and increasing the amount of lead directly reduced the wear rate. Lessons Learned  -The additive function of lead naphthenate is a combination of physical absorption through rubbing or pressure and chemisorption. -Depending on the mechanics in the contact (sliding versus rolling), the effect of temperature had a significant effect. This appears to indicate that higher concentrations of lead would be required for more severe applications involving pure sliding and/or high temperatures as the lead is consumed faster through reaction with surface layer steel oxides and sublayers. -In rolling and mixed contacts, lead naphthenate creates a strong tribofilm on the steel surface that aids in protecting from wear but at the same time will increase the friction in the contact. -The formation and durability of lead naphthenate tribofilm is dependent on the environment with higher performance coming under vacuum conditions. It is also believed the lack of oxygen promotes this life due to the lack of oxide formation on the steel and degradation of the lead naphthenate. -In general, an increase in lead content will decrease the wear rate. Acknowledgements  I would like to thank the team at Nye Lubricants specifically Craig Deschenes for providing the samples of  the lead based Pennzane, Allison McGlynn for the MTM testing and profilometry, and Paul Moses for  running the SRV testing and profilometry.   NASA/CP—2018-219887 80