HI, I am glad to help you.
There are a few things that can cause this. I will list them for you .
1. First, DO NOT use any multi-grade oil in these engines ! Use ONLY Sae30 oil. Multi-grade oils will cause engines to burn oil and it will wear out the engines MUCH faster.
The only reason that the owners manuals will tell you that "10W30 is an ACCEPTABLE substitute" is because the Government says that you have to ell customers that they can use oil that is available in 100% of the United States and Canada ... and there are some places in the United States and Canada that you cannot get SAE30 oil... Mainly in the Northern states and Canada. You can use 10W30 if the Temperature outside is below 25 degrees... but ONLY then.
2. A dirty air filter will cause the symptoms that you are talking about.
Air filters should e changed AT LEAST every 15 hrs. Yes I know that the owners manual says it can go longer, but take my word on it, the dirty conditions that these mowers are run in make it so that the filters must be changed much more often. Change the air filter.
3. If the oil and filters were not changed after the first 2 hrs and then again after the next 5 hrs, it is possible that the engine may have wear caused by "dirt ingestion" this is because it is absolutely necessary that you get all the dirt, and grit and metal shavings that were left in the engine when it was machined.... as well as the shavings caused by the break-in period,.... out of the engine..... otherwise it is like running the engine with sand in it.
4. The engine might have overheated at some point in time and cause some damage. Sometimes the overheating is caused by grass that may have accumulated under then engine cover from use, and also you could have a mouse nest under there cuse because mice like to build nests inside these engines.
5. You might have a valve guide that might be worn, but it is not likely.
6. A head gasket that is blown can cause this.. Head gaskets can blow from over taxing the engine and also form overheating as described above.... and from the use of 10w30 oil
So what do we do.
First change the oil and oil filter to SAE30 oil
Next replace the air filter.
Next, remove the engine cover and clean out any debris that might be under there covering the cylinder and such.
That is about all you can do for now.
Feel free to contact me here at this question anytime ...even after you hit the "ACCEPT" button ...and I will try to help further.