Just purchased a Fastrak 52" with 20 HP Honda. The Hustler manual says change the oil after 5 hours use and the Honda manual says to change it after 1 month or 20 hours. Which is correct? I purchased Opti-4 oil from my dealer to run in the mower which will extend the warranty for two additional years. I noticed he put in SAE 30 and the manual calls out 10W30. Is there that much difference and I am wondering if it will void the warranty in anyway?