The best oil is the one that actually gets checked once a month, or some would say every time you fire up the engine. Better yet, the oil could be changed every other year, or even every 5 years. I am using 10W-30 in the mower and also the snowblower. Both are 4 stroke engines. The mower is 6 or 8 years old, and the snowblower is 5 years old. Both run fine.

The oil is specified in the owner's manual, and typically will be 10W30 or SAE30HD. I believe in using only oils that are certified for 'air-cooled service'. Generally, you would use the lighter of the two depending on the oil temperature at start-up. Some manuals allow 30 weight down to 32F. Since very few mowers are started at that ambient temperature or lower, the 30W would be good for those. Honda, for example, typically calls for 10W30 at 50F or below, and allows 30 for temps above that. Again, the key is the oil temp at start-up, which may differ from the ambient temp depending on how the mower is stored. If it's kept in an attached garage, the oil temp may be 50F or higher. If it is stored in an unheated outside building, the oil temp would be assumed to equal the ambient temp after a few hours of not running. I have two mowers - a Tecumseh engine and a Honda engine. I use 10W30 in the Honda and 30W in the Tecumseh since that's one that allows 30W down to 32F - I can't think of a time I have needed to start a mower at 32F or lower.
I might be behind the times on this as newer mowers may well spec 5W30 or even 5W20. All you have to do is to go by the manufacturer's recommendation for the climate you're in.
As far as which brand to use, you can't go wrong by using the manufacturer's branded products, like Briggs & Stratton or Honda oil in the appropriate weight. That is not to say there aren't better-quality oils out there, like Amsoil '4-Cycle' which is 'air-cooled' rated, or Opti-4 in 10W30 and 30W.

The manufacture has a whole raft of engineers who are paid good salaries to determine the best motor oil for their engine. Check the web site for your particular engine mfg. and use what they recommend. Why second guess them?

Note that the equipment mfg. does not always recommend what the engine mfg. recommends.

Walt Conner

Briggs and Stratton recommends Synthetic 5w-30 or 10w-30 in all their engines in all temperature ranges.
Here's the quote:
"We have recently modified our engine oil recommendations to state that you may now use a synthetic 5W30 or 10W30 oil in all temperature ranges."
Key words: (Use their Synthetic) . Multigrade dino oil will be consumed at a higher rate according to Briggs.

Here is a link that might be useful: Synthetic oil.

I use Amsoil 5-30 Synthetic on Lawn Mowers and 20-50 for Air Cooled Generator Service. However any Manufacturers Syn - Grade oil will provide the required protection . Briggs Toro and Honda are current Engine Suppliers who provide such Premium Oils as RDaystrom has advised. Premium Grade Dino Oil of the proper Viscosity for the ambient operating conditions would be my alternate choice , however intervals for oil changes would increase accordingly .