The first thing you should do is take the 19hp motor off and pitch it. Then for around 1200$ you can get a 23hp. Takes about 20-30 minutes to change out. I recommend two people ,the bolts under the deck for the motor mount and the clutch can suck.
Either way the thing wont last much more than 24-2500 hours no matter what, but the cut will be much better.
I run fully synthetic and change my oil when i remember or around once a monthish.