^^ DXM is an opioid and in my past experience it does potentiate opiates. it is absorbed through the CYP2D6 enzyme, the same cytochrome that most opiates are absorbed through so it sort of acts like GFJ or tagamet if you take it before as it will occupy the enzymes so less of your oxy will be metabolized and excreted. as i stated before as well as being a NMDA antagonist, it also has some, but not much affinity to bind to kappa, delta, and mu opioid receptors.
i personally absolutely detest taking anything past 60mgs of DXM or any dose of DXM that makes you 'trip,' but when taken in low doses in conjunction with an opiate such as oxy, it adds great warmth.
finally because it's ability to antagonize NMDA receptors is so poor, it is not the best choice for reversing tolerance. memantine is 10xbetter at reversing tolerance because it has a high affinity to bind to these receptors.
however, justin does make a good point about mucinex. i wouldnt suggest taking much as it contains other active ingredients which arent good to take in large doses. Delsym's only active ingredient is DXM so that is what i prefer