I mowed the lawn once while the greenies were having a meet in the local park - they were not happy

With regard to 4 strokes, I use regular car oil. I tend to use 15w40 cheap oil (the $8-10 per 5l on super special Gulf westerns or Nulon types) for a few mows after I get a mower. That strips the muck out.
After the few mows, I drain it and put in Magnatec 10w30 or GTX, or any other good 10w30.

However, I change the oil before it gets dirty - probably a good idea for the average person (who only changes it once every 2-3 years or more) to use the non detergent so the dirt remains at the bottom.
On that note, what ever happened to flushing oil? I got a bottle of Nulon engine flush for free, but haven't used it. Don't know what it might do.