I also have have the 50" (2015), but with Exmark-Branded Engine, and had an interesting conversation with Support on oil that I'd like to pass on...
Seeing that my dealer is greater than 50 miles away.... And I have no trailer to haul it on... When I bought my 50" Quest Steering Wheel unit, I realized that I would be doing the oil and Hydraulic fluid changes.... So I wanted to be confident that my actions would be the best for mower and warranty...
Regarding Oil changes, I spoke with support and they suggested at the 5 hour oil change to NOT go Synthetic, but remain using good quality non-synthetic oil... At the next (50) hour oil change..... THEN Switch to full synthetic Motorcycle oil if access to Xmark oil is not available... They prefer Xmark Oil filters...
Hydraulic Oil and filters is another story... Xmark Support strongly suggests using theirs, and I heard about that at the purchase... So, I purchased 2 changes of Hydraulic Fluid and Filters for both oil and fluid... with the unit as part of the original purchase....
And dealer will ship them if necessary...