Some of the older manuals for Gravely/Ariens indicate the "recommended oil" is the Gravely 00057100 Synthetic, which is
really M-1 15W50. This does not mean that the ZT-3100 comes with that oil fill from the factory.
So now I am really intrigued about the Castrol 20w50 reference. Maybe there is a difference between Hydrogear's preferred factory fill, and the ZT equipment manufacturer's preferred fill.
The Gravely manual does state the factory fill is a 20W50, maybe that is a non-synthetic Castrol or M-1, used as a break-in oil.
Tundraotto, if you have more info on this, please share.