A couple of thoughts. On the Ferris site check under the tab for contacting the company and send the service department an email asking about the information you need. I have never delt with Ferris but several other equipment manufacurers,( eXmark and Echo among others) have been very good about replying to questions. Most Kohler engines do well on SAE 30W oil.
Good luck finding the info you need. Sometimes it is a maddening process.