I thought that the oldest running node would be after the hard fork in 2013, so the oldest working node would be starting from 0.8.1.
So these will be the oldest running nodes
https://bitnodes.io/nodes/?q=Satoshi:0.8.1Bitcoin Qt 0.8.1 helped fixed the problem. But BIP 50 explain why you still can use 0.8.0.
In order to restore a canonical chain as soon as possible, BTCGuild and Slush downgraded their Bitcoin 0.8 nodes to 0.7 so their pools would also reject the larger block. This placed majority hashpower on the chain without the larger block, thus eventually causing the 0.8 nodes to reorganise to the pre-0.8 chain.
Note that bold added by me.