Q: There is a small brown sludge oil layer in my coolant tank.

Often times, sludge in the radiator is an indicator of a leaking cylinder head gasket, which can lead to major engine damage if not checked out soon. A blown head gasket may cause a number of different symptoms. A head gasket that fails between cylinders will generally cause a misfire and potentially compression leaks from one cylinder into another. Low compression will eventually result in a rough idling engine. When the head gasket fails between a cylinder and the coolant port, coolant may leak into the cylinder causing it to be burnt with the fuel charge in the combustion chamber. This often results in misfires on startup, especially after the engine is run, turned off and restarted and in some cases may result in the engine not maintaining enough cylinder compression to even run or start. You may also notice a slight bit of steam like smoke being produced from the exhaust as a result of the coolant being burnt inside the combustion chamber along with the fuel charge. This type of failure may not always show with a compression test. I would recommend having an expert from YourMechanic come to your location to properly diagnose the symptoms you describe.