The oil filter bypass opens when the pressure in the oil filter increases past a certain psi. I have had it due to metal particles in the filter and the checklist has you retard the thrust lever until the message disappears, if it does not disappear the engine has to be shutdown. The emergency would have been declared as the aircraft could not maintain its assigned flight level on one engine and would need to descend. However I would not call the subsequent landing an emergency landing. To me an emergency landing is when there is difficulty with the aircraft handling or some doubt the aircraft can be landed safely and stopped on the runway without risk to passengers. One engine running at idle is hardly an emergency landing.