Breather / fill cap is on the top of the transaxle.
Service manuals shows John Deere Plus-4 10W30 Engine Oil, Class CD as the oil for it.
Copied below from the manual, sorry the pictures wouldn"t go with it.
Fill reservoir with 1.6 L (1.7 U.S. qt) of John Deere
Plus-4, 10W30, Class-CD engine oil. Oil should
reach bottom of fill hole (D).
4. Install breather valve assembly (E) and fill cap (B).