As above posters have stated, you're almost certain to be alright given you're supplied dosing history. I'm in the camp that an opioid is an opioid and while there are some variables which certainly mediate the danger of a given drug, it's mostly dose dependent. Hydrocodone, oxycodone, hydromorphone, fentanyl, oxymorphone, hell, even codeine can kill. You just need to know your own limits and tread lightly when combining CNS depressants, even if they are both opioids. The thing that, I feel, gets users into trouble frequently with methadone is its deceptively long half-life and accompanying onset and duration of effect. If you're used to oxycodone, with a 30 minute onset of action from a PO dose, the 3 hour - or longer - onset of methadone can trick you into thinking you have underdosed and potentially into consuming more. Although, I have heard just recently that methadone causes respiratory depression at a higher rate mg for mg than many common opioids, but I have no references to back up this claim at the moment so check it out for yourself.
Effect: I'm not sure if it's a regional/european difference, but some people on here tend to use the abbreviation ml/mil or what have you in place of mg to indicate milligram. This confused the shit out of me at first too as I'm sure you know as well (being on methadone) that most of our juice in the states is the 10mg/ml stuff. I just go off of the assumption that that is what most people mean.