I used/ abused methadone for over 8 yrs and it should not be effecting whether you feel oxys, I know I used to take roxi 15s and 30s with the methadone here and there, and I definitely could feel them.
Methadone is a strange animal, Ive heard more than a few people say it doesnt effect them or get them high, but it sure got me high, as little as 20 mg would get me feeling great for hours (usually around 6 hours), and if I took around 40-60 mg, I would be really high, nodding, no aches or pains, euphoria, motivated as hell to do just about anything too!
Plus, tolerance to methadone takes YEARS to build up, some people can be fine on the same does for 5 yrs or more, but it will KILL your tolerance to other opiates, and usually have to wait about 3-4 days before even have a chance to feel another opiate.
I would strongly advise on taking methadone for too long though, the w/ds are absolute hell, and you will basically be bed-ridden for weeks or months, its almost impossible to come off it on your own, as once you start into the w/ds, you WILL go out and cop something to help, if you have a connection and any amount of money, it is nearly impossible to resist the urge...thats how bad the w/ds are,just keep this in mind before enjoying the done for too long, Id almost advise getting a heroin addiction versus methadone!!