I only noticed a difference in strains within the first month or two of using kratom. After that, it was more a matter of whether the kratom was good quality or not, because after a while, at the end of the day, it always does the same thing. Green Malay is known for being a well balanced strain, meaning it can be relaxing and energizing. Really though, with almost all strains, the desired effect is dose dependent. Lower dosages, 2-4g usually provide a more stimulating effect, whereas higher dosages 5g^ become more sedating. This is all relative to your individual metabolism though. I'm sure some people would get rocked off of 2g, whereas others would need closer to 10.
It's also important to know that more kratom is not always a good thing. Unlike other opioids/opiates, kratom doesn't really cause repiritory depression (although I'm sure if mixed with a handful of CNS depressants someone could achieve it), and a kratom overdose is incredibly uncomfortable, much like a caffeine overdose. Your eyes get really wobbly, and your hands start shaking uncontrollably. I've put myself through that multiple times and it sucks.
As for Kratom's relative strength compared to other opiates, there doesn't seem to be much research on the matter, but looking back, I think that while kratom has alkaloids that are mu-agonists, it also has a lot of other alkaloids that add the the experience. Some of these serve as muscle relaxants, etc. So while there is definitely a cross tolerance with the part of the plant that affects the mu-receptor, it's not as sever as with straight up opiates. To illustrate this point, I started taking kratom to get off buprenorphine, I had been plugging 2-3mg a day, and I took 7.5 grams of malaysian kratom and it fucked me up. Don't get me wrong, I wasn't nodding, but I got an intense euphoria that I haven't experienced from regular opiates in years. Granted, after a week or so this euphoria definitely started to wane, but still, I was impressed.
If looked at as a conventional opiate, than yes, kratom is comparatively extremely week by weight comparison, but when you consider that it's dosed in grams, it's really not. Not to mention, of those grams, only 2% (or some ridiculously low number like that) is actually active alkaloid. Someone on here did an acid/base extraction on kratom a while back and I believe he said that with the resulting extraction, it was active in dosages as low as 20mg or so.
Anyway, I'll end this rambling now, but I'm fairly certain that kratom should take you out of oxycodone withdrawal, of course, depending on the quality of the kratom (head shop stuff is often not that good, but TBH I've had it a few times and certain brands aren't terrible either).