The fact that it's the entire bank is usually caused by a bad O2, but since you changed it, that's a tough one. I though the intake gaskets where all individual per runner. That would be mighty odd to have all of them only on one side fail. If it was oil contaminating his plugs through the intake manifold, it would also not only be one entire bank, and he would probably experience detonation or a miss fire in that case. Are you maybe burning coolant or oil? I suppose it could be oil seals, it maybe be a head gasket too. It's hard to diag. without seeing it.