Shell Rotella T1 straight grade oils still contain all the zinc that they had before the change in specification.
Shell's Triple Protection multigrade Rotella line also has plenty of zinc in it; far more than most oils formulated for gasoline engines but using a multi-grade oil in temps above freezing is not recommended by Kohler.
Now if you want to spend more money because it makes you feel good to do so, then you can run the Amsoil product that is labelled as being suitable for running in small air cooled engines or choose oils labelled by Kawasaki or Kohler and pay a premium price also.
The Kohler K and M Series engines are both splash-lubed and have no oil filtration. Therefore, it is essential to drop the oil every 25 hours of operation MAX to get rid of the contamination. If you are really concerned about extending the life of your K or M Series engine, then change the oil every 15 hours.
What matters most to these engines is CLEAN OIL, not fancy, shmancy expensive oils that promise long oil life. Whether you spend 3 bucks a quart or 30 bucks a quart, ALL oil will become contaminated at the same rate and without an oil filter, changing the oil frequently is the only protection against rapid wear.