Jason,
Ask 10 people this question and you will get 10 different answers (and opinions). It would help for us to know what your engine is powering, your climate, and your operating conditions.
I have the FH680V in an Exmark Zero turn radius mower at my workplace and I studied the oil question a bit for this application.
I'm pretty much a "by the book" type of guy and generally use what the owner's manual recommends.
The Kawasaki owners manual lists 5w20, 10w30/10w40, 30, and 40 weights on their temperature chart with the following note:"Using multi grade oil will increase oil consumption. Check oil level more frequently."
Hand written next to the chart in my manual: "Called Kawasaki
on 5/07.....They prefer 30 wt. first, 10w30 second."
Exmarks "opinion" on their faq page:
Question: What type of oil should I use in the engine?
Answer: Use what the engine manufacturer recommends. Be aware that Kawasaki ® still recommends straight 30-weight oil in their engines, and Kohler Command™ engines require multiweight 10W-30 oil. The man who hired me here at Exmark, over 15 years ago, always said "the cheapest thing you will ever put in that engine is oil.” What Lou meant was that changing the oil on a regular schedule, even if I have not hit the number of hours the manufacturer recommends, will extend the life of the unit. Some people ask about using synthetic oil in the engine. It is OK to use synthetic oil, but you should not start using synthetic oil until you have at least 250 hours on a new engine. The other thing to remember is that using synthetic oil does not change the recommended oil change interval. "
250 hours before synthetic???? Take their opinion as you wish.
Finally, the guys at lawnsite.com also generalize that Kawasaki's "like 30 wt. and Kohler Command "likes" 10w30, but you will find their group using all sorts of stuff, even 15w40.
As mentioned above, the Amsoil ACD or ASE would eliminate all guess work since it meets all the criteria....its a 30 wt, a 10w30 wt., AND a synthetic....no decisions to make!!!!
I've been using Rotella HDEO 30 wt. with success. Good luck with your decision.