Oil turned black in a day

[cowofawesome]

I picked up a dead Murray walk behind that I'm trying to get running again. I filled it with new oil today, but I grossly miscalculated the oil capacity and it ended up burning a ton of oil when I started it up. I drained it out to a healthy level, but all the oil is dark dark grey with black specks in it. Is this probably just because it was burning or could there be another problem that would make it go black after running for 10 minutes?

Also, when I turn it off the muffler/exhaust smokes lightly for about a minute. Is this OK or is there something I should check for?
Thanks

 

[ILENGINE]

The smoke coming off the muffler is probably residue from setting, and the oil in the muffler. You probably don't know the history on the mower, so there is a chance that the oil has never been changed, and there is sludge in the engine, which the detergents in the new oil started to pick up.

 

[cowofawesome]

You were right, I changed out the burned oil and let it run for a while, it's not smoking anymore. Literally every surface of the engine was coated with black sludge when I got this mower, and there was even a nice buildup of it inside the carb, so it makes sense that it just had to burn all that junk out. Seems to be working fine now.
Thanks!