mercury 9.9 4 stroke questions

[whiskerwhipper]

I have a 2007 mercury 9.9 4 stroke. It has probaly at least 8 hours or 10 hours on it (still in break in period) How long can you run the motor before having to change the oil in it? Does really say 10 or 20 hours in the manual. Recently the motor puffs out a lil smoke after i start it up and it stops after it gets runnin. That bad? I might dump out the fuel i have and put in fresh fuel to see if it cures the problem. The fuel i use now was pumped back in January to fill up my tank during storage but I added some Startron enzyme stabiliser in it to keep things fresh ( me thinks). Many thanks