I have a 1991 Mercedes Benz 300 SE I had noticed a creamy substance in the coolant tank I flushed the coolant system out till clear water came out of all ends then refilled the system 1 week later the creamy substance has returned if it is a head gasket can I use a head gasket sealant and how log would that prolong that from happing again, If need be to replace head gasket how involved is the head job? I've torn down many motors in the past is it difficult to do a gasket on the Mercedes, Thank you for your time!!!