I'm shure its not corrosion because its a dark brown colour. I am running 20% nitro with 12% lubricants. I'v been using this fuel for a long time and never had this problem...I think I might use after-run oil even if it recommends not to. I will take apart my engine and inspect everything and see how its doing....This is my stock RTR engine and soon I will be getting the EPIC .18 soon and then I might switch to O'Donnell fuel. Thanks guys