Simple datsun exhaust systems don't hide the water build up in the system. If you had bigger mufflers, the condensation buildup would evaporate better, or not get blown out the back. You might also have a slightly leaky headgasket, letting coolent get into the engine cylinders when the engine is hot. The richness is what is leaving the black soot in the exhaust, making the water black and oily looking.

Chris