Well, I'm rebuilding my 350 and I was removing the radiator and I had some brown sludge in the goose neck. I then drained the fluid and it was green like normal then as it started to get empty started spitting out dark fluid and by the end it was very brown. I removed the hoses from the engine and rad and there was brown all around the ends. Anyone know what could be causing this? I think it's oil mixing with it but maybe you guys know what it is and where it's coming from.. thanks!