The Enchiridion of Indulgences provides:
The faithful, who devoutly use an article of devotion (crucifix or cross, rosary, scapular or medal) properly blessed by any priest, obtain a partial indulgence.
But if the article of devotion has been blessed by the sovereign Pontiff or by any bishop, the faithful, using it devoutly, can also gain a plenary indulgence on the feast of the holy apostles, Peter and Paul [June 29], provided they also make a profession of faith according to any legitimate formula.
While there was at one time a plenary indulgence attached to wearing a crucifix with a St. Benedict medal attached to it, it appears that this indulgence is no longer in effect. Instead the indulgence attached to crucifixes in general would apply. That being the case, you can rest assured that your regular crucifix is sufficient, but you can certainly obtain a crucifix with a St. Benedict medal if you wish.
In any event, please be wary of reducing the effects of an indulgence to a “magic formula” that is supposed to ensure salvation. Indulgences can only be applied to forgiven temporal sins (thus necessitating, under normal conditions, sacramental confession) and the grant presumes that one has the proper dispositions to receive the indulgence. The requirements for obtaining a plenary indulgence are:
[T]he execution of the work, the fulfillment of the three conditions [sacramental confession, eucharistic Communion, and prayer for the pope’s intentions], and that full disposition of spirit which excludes all attachment to sin (Norms for Indulgences V3).
Primer on Indulgences
Myths About Indulgences**