Chain replacement which one?

O & X ring simply refer to the shape of the seal between the side plates and roller plates in the drive chain. The marketing claim is that X chains last longer the O.

If you cut an O ring seal in half the cross section is an O and an X is, yes you guessed it an X.

An X-ring is "supposed" to help keep the lubricating juices in and have less friction. BS most probably.

a broken chain can also whip around and smash your engine cases which will break like candy!