Small Engines (Lawn Mowers, etc.): Bolens 4.65 horse lawnmower, little holes, strange type

Question
I have a Bolens 4.5 horsepower lawnmower that I recently purchased second hand. I have for the most part had good luck with it but it did give me some problems at first until the prior owner was nice enough to clean it up and get it running again.I don't really know what all he did but it has run fairly good since then. I noticed today as I pulled it back out for the season that the oil stick was coated all the way to the top, after a brief use, with oil I'm assuming that was very, very black in color and a strange type of viscosity to it. I was wondering if this is normal or if there is something wrong.Once I cleaned the old oil off the stick and re dipped it it was at level but again very black in substance. I also would like to know where to purchase a filter the one on the outside with the little holes in it?? Does this type of engine have to be serviced periodically as it would not start or run until the prior owner completely cleaned all the working parts and then it has ran fine for part of a season and now into the next. I also am having problems with a clamp type part that fits into a tiny hole on the arm of the mower that allows you crank the mower. It keeps coming out of the hole and it is annoying. I was wondering how to fix something like that?? Also the bottom catcher is dragging the ground when I mow and is annoying also. Can this piece be removed so that the mower will navigate better?? Any assistance would be appreciated.

Answer
The Bolens name was purchased by MTD, so your pushmower is
likely an MTD unit, so they are fairly common.  If you could find the tag on the mower deck and give me the model number, I could
look up the blowup and know for sure which model you have.
Also, if you have any numbers off of the engine, that would help me also.

  In general, I would recommend dumping out the old fuel and oil,
and replace it with fresh.  Push mowers generally do not have an
oil filter, but all have an air filter, and many have a fuel filter,
and all have a spark plug.  Taking the air filter up to a chain store
like wal-mart or lowes, you will find a replacement, as well as the plug and other items.  Replacing the oil, you should use 30 weight,
or sae30, never use 10w40, which cooks down to a black smelly goo like you have described.

 If you could get back to me with some numbers, I can focus a bit
more on your other questions