Dick from Canada
Changed the oil and  the spark plug in my CorePower ,model 58070 last week .  Used a 5- 30  synthetic ,same oil as used in the past. This morning during it's exercise there was blue smoke on start up and the engine was missing badly . Smoothed out after about  a minute. Checked the oil level and not over filled .   Use a different grade perhaps like 10-30 ?  Leave the synthetic and go to  a conventional oil perhaps? This unit has about 50 hours outside of exercise time and is too young to smoke .
Most likely a little oil accumulated in the intake from the breather. 
Keep an eye on oil consumption, and if it's not excessive, I wouldn't worry about a bit of smoke on startup. 
When you pulled the old plug did you notice oil or carbon deposits?