I've been working with Lund on a tune for the race gas I'm running in my 2016 Mustang GT. The gas in question is Sunoco 260 GT Plus 104 unleaded race gas. I had been asked to re-gap my plugs as part of the tuning. When I removed the plugs from the block I saw orange all over them. Let's find out why.
The plugs were very orange as they came out. I had known this was likely to happen because of similar coloration on my oxygen sensors. Many unleaded race fuels contain MMT, also known as methylcyclopentadienyl manganese tricarbonyl. See why it's called MMT now? Hah!
So this stuff leaves orange manganese deposits all over spark plugs and exhaust components. Per Sunoco it is not harmful and I haven't seen any negative effects. I don't think I'd do this in a daily driver, but I don't think it's a huge problem for the Mustang.
I did wind up cleaning the spark plugs up a bit, just because they were out and being re-gapped. I also borescoped the engine just to check out how things look inside the cylinders, looks good!