The side plate thickness does vary as you go from regular duty to heavt duty chain, but a duc is heavy duty specced anyway.
The only difference between an o-ring and an x-ring is shape
The x-ring is better because it gives a pair of sealing surfaces, and hence is better at keeping the original chain grease inside, and is slightly better at keeping cack out ... as long as you keep your chain clean(ish) !
All this adds up to less adjustment required
