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No fuel additives but I did use break-in oil after the rebuild which contained added zinc and phosphorus I believe. Would that cause this red residue?
The cars done around 3000 miles and two oil changes since then though.
I thought I remembered hearing/reading about certain octane boosters causing red soot like that in the exhaust. Those bottles you can add, or certain higher octane fuels would cause it.
This was years ago that I heard about it though (like late 90's), so YMMV.