Smoke Rings

It felt like I have had this lawn mower forever. Reality is, I've had it since 2006. No repairs or maintenance ever. It's always run like a champ until recently. It still starts up and runs, but seems to lack the power that it normally has. It seems to run slower and quieter. I think it's not cutting quite as well, due to the slower engine speed, and I seems to push it along much slower to compensate. I guess its time to tinker and see if I can fix things.

Step 1: Check air filter. It's really just this sponge kind of thing. It was covered with greasiness on one side. I cleaned it up.
Result: I imagine that the engine is running smoother, but notice no difference in the power.

Step 2: Check the oil. When you open it up, and find that's there is no oil, that must be a sign that it needs some. I pour in the amount that should fill the tank with 10w40.
Result: The mower does seem to be picking up a bit more power gradually. Hopefully as the oil spreads along it will continue to gain this will solve my problem.

Step 3: Change spark plug. The original plug was a RJ19LM, and I got a J19LM instead. I was told the only difference is the later does not have a resistor to tone down radio interference (something I wouldn't need)
Result: No noticeable difference

So that's where I'm at right now, except for that when I poured in the oil, I put in the amount of oil I'd need to put into the empty tank, not thinking that although the dip stick showed no oil, that they would be some below the level of the dip stick. So I've overfilled. Dark smoke is sometimes just billowing out of the mower (is this the expression burning oil?) and sometimes the mower expels some rather interesting smoke rings.

I'm going to try running the mower a bit and see if the new oil circulating helps my power problem. If not I'm not sure what I'm doing next. It could need some kind of tweaking, but the manual suggest not to make adjustments to the motor yourself.