Regarding the FEP encapsulated silicon type o-rings: The FEP is the clear outer layer and the silicon is the red part in the middle. The good news is that the FEP layer is pretty tough against hydraulic fluids and synthetic oils. The silicon is in this case essentially just a filler since it has very low resistance to oil/hydraulic fluid. Another plus is that the upper temperature range is much higher than what your pump will ever see. However, I'm curious to see how those rings hold up over time since they are so much harder than the originals.
You can also use either NBR ("Buna-N") rings which are as you pointed out *probably* what's in there to begin with, or HNBR rings which have a higher operating range. Based on the way your rings broke I'd say that heat is an issue and go with the HNBR rings. McMaster sells them in bags of 25-100 for about what you paid for single rings. If you look around you might be able to find smaller bags or even individual rings for a lot less.