There are various oils to condition wood (lemon oil, etc), but they're just for keeping wood from drying out, etc. They are not wear protection. Nothing like a conditioning of the wood is going help your situation. There are no surface-patching compounds for wood floors that will tolerate a chair or anything like it from what I've seen. You have 3 choices as I see it. Rug, office mat, sand and refinish the whole floor. I'd also include a non-sanding resurfacing, but it sounds like your floor is way past the requirements for that (it's a maintenance thing to be performed regularly, and you're beyond that if your floor is brittle and pretty much bare wood with no finish on it now).
Remember that if you put a rug or office mat down, you must lift it up and clean under it every few weeks where chair is or you will just start sanding your floors with the dust/grit that builds up over time underneath the rug/mat.