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Great question – sorry it’s taken a little while to answer but I needed to do some research. The reaction of sulphur-based compounds and ammonia isn’t an issue that I’ve come across in the past. In theory, I think there’s the possibility of polymerisation reactions that yield red varnish compounds.
Although there isn’t any direct research into this area that I could find, there is an interesting paper available at the link below in which they look at the reaction of Ammonia with Sulphur Dioxide. That yielded (HNSO)<sub style=”background-color: var(–bb-content-background-color); font-family: inherit; color: var(–bb-body-text-color);”> x </sub>polymers, so I can imagine a similar reaction in sulphur-containing group I base oils.
https://www.tandfonline.com/doi/abs/10.1080/03086648008078204