The low viscosity of warm oil is normal, sparker. That is why you always warm up the engine before oil changes. I do not see any signs of water in that oil, just carbon. Most likely the bottom of the oil sump is pretty dirty, and your series of oil changes didn't affect that much. Does the engine produce any blue exhaust smoke? If not, and if you can feel clear, but not intense, compressions on the starter cord, chances are all is well to keep using the engine.
I wouldn't put that oil back in the engine either. The only time I re-use black oil is when I know for certain that it has only run a short time (up to perhaps 8 hours in a worn engine), and then I use it for running-in oil (the first two hours after fitting new rings). Then I change it, and if it is a Honda at least, it will stay clear for quite a few hours. However if you have black crud in the bottom of the sump, your oil will always get dirty fairly quickly just by picking up old garbage.
