A brown oily substance has appeared in the fresh water coolant tank.  Actually has the consistency of taffy in leaving a trail behind when you touch it.  Assume it is oil mixed in with coolant.  No water in the oil side of the system, just the coolant side. 

 Head gasket?  Hoping this is the least catastrophic explanation.  No steam coming from exhaust and the engine runs great.  Running temperature is completely normal.   Spirit Borne had been stored for six months in a yard awaiting launch post Irma, the longest time she has been put up in 19 years.  Engine has about 6,000 hours on it.  Being in the Caribbean it is not as if there were wild fluctuations in the temperatures.

Where are the drain plugs for the 4236 so I can start the process of flushing?  Someone recommended Gunk engine degreaser run through in a mix several times.  Can you use the hoses attached to oil cooler to run cleaning fluid through the engine.  Any suggestions welcome.