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Just got off the phone with him (Uncle). What a conversation. My head hurts.
Although not an M.D., his input; reference your question, was that compounds like ZDDP (Zinc-Dialkyl-Dithio-Phosphate) contain both elements (sulfur and phosporus) that can form compounds under exposure to ICE service conditions that can eventually cause health problems especially under conditions of long term exposure even at what some would deem as safer, lower concentrations. The sulfur compounds formed within engines that are relaesed into the enviroment like hydrogen disulfide (H2S: hydrogen sulfide, smells like rotten eggs) can form various acidic compounds, essentially sulfuric acid (H2SO4, when exposed to and allowed to react with atmospheric moisture) that can be traced as causal to respiratory conditions like asthma that seem recently to be on the rise. He noted that as sulfur content in fuels decreases (especially synthesized diesel fuels with no sulfur and very high cetane index numbers) that sulfur content from within AW additives such as ZDDP that degrade under service conditions begins to have greater importance as a source of sulfur (-Dithio) and phosphorus (-Phosphate) compounds in exhaust emmissions (nanoparticulates) that are typically composed of microscopically fine (nanoparticles @ 30-500 nm) Poly Aromatic Hydrocarbons (PAH's) that serve as both nucleation and accumulation sources for SO2 (Sulfur Dioxide) as well as the various phosphorus and NOx (nitrogen oxide) compounds emitted by an ICE. He also added that most quality oils like Mobil 1, Pennzoil, Castrol, etc., etc. have plenty of AW additives in their formulations and that adding treatments like STP Oil Treatment were largely a waste of money as the AW/EP already present in the oil's formulation was adequate and adding (more) ZDDP was an exercise in diminishing returns since the existing AW/EP additives already present in the formulation would be sufficient until the next lubricant change.
Certain phosporus compounds have been shown to effect bone health causing embrittlement and other degenerative issues and the commonly encountered H2S and SO2 can cause respiratory issues and have even been loosely tied to increased pulmonary (lung) carcinogenic rates.
The other reason for abandoning ZDDP recently is that Zinc (Zn), Phosporus (P) and Sulfur (S) are all 'catalyst poisons' that tend to destroy the platinum or palladium catalyst applied to the ceramic honeycomb substrate of an automotive catalytic converter over time because ZDDP degrades under thermal stresses in an ICE liberating these elements from the ZDDP molecule and their exclusion as mandated by the EPA is meant to address those concerns. The environmental and economic considerations (as the price of metals, especially platinum and palladium which were not inexpensive to begin with, has gone through the roof as we all know from rising ammunition prices) seem to be the primary motivation for these changes as prior to this, using ZDDP was considered an inexpensive and effective way to achieve AW qualities in a lubricant. No more the case.
As for my humble input, regarding the use of lubricants containing ZDDP (the AW additive that is found in STP Oil Treatment) on your guns as a lube, would be that it most likely would NOT be an issue, due to the fact that the conditions (heat and pressure) requisite for such thermal degradation (wherein the Zn, S and P are chemicaly liberated from the ZDDP molecule) and nucleation (formation of ultra-fine nanoparticulates, especially in the 30-50 nm range that could support accumulation) are simply not present in firearms and most risk (if any) could be sufficiently attenuated by simply washing your hands after handling such an additive. In other words, I think the risk is extremely low and wouldn't worry to much over it unless I were to gulp it down by the bottle, which is highly unlikely.
Hope this provides some perspective.
