I always say RTS 5W-40 for it's price to performance ratio. This season, I ran M1 15W-50 EP in all my stuff and I have to say I was very impressed. If you want the very best protection and money is a factor Rotella 15W-40 or Castrol HD-40 are both excellent.
If you're just changing oil on a push lawnmower, keep in mind it takes less than one quart so a single bottle will get you through an entire season - with top offs - so you can splurge a little on the oil quality.
Honestly though, if you run any high quality diesel oil that's optimum protection for your small engines.
Heck, if you run ANYTHING that's changed as often as once a year you're beating 95% of small engine owners.