Pro Turn 60 Hydro Oil ZT3200

What Oil am I supposed to use? The dealer told me to use 20w50 but my Mower manual says to use 15w50. Just searched online and found here http://ift.tt/1eoioOB that it shows to use the 20w50. Which shall I use? Also anybody have experience with using Synthetic oils? Good or bad for these Hydros?

Thanks