Engine coolant in 2010 Shelby Gt 500 or older

[beast]

Any one owning a 2010 or older Shelby Gt 500 with cast iron block have any problems with the engine coolant plastic reservoir tank getting a brown slime coating on the inside of it? Took it to my Ford Dealer service and they said nothing to worry about, that it is a little rust build up in the tank. The actual coolant is still clear yellow no brown in it. They also said that when the engine is built that they put a small container of sealer in the engine coolant to coat and seal the walls of the engine and that may be what is staining the tank. They claim it is not needed at this time to flush the engine either! Any one have any suggestions other then having the coolant tested or flushed? Thanks Beast

[aonebadbone]

I don't have that in my tank 07 with about 14000 miles on it