6-8 mg is plenty for what? I'm sorry but its still a pretty scant amount for no tolerance. You say you have done codeine, well 60 mg of codeine is about 5-6 mg of oxycodone, to give you an idea. Since you have taken codeine you can base it off that. IDK what everyone is so concerned about here, all are preaching don't break the time release but guarantee put each one of them in your shoes and they do the exact opposite. Your not going to OD on 20 mg of oxycodone. And unless you got a real weak stomach you won't be throwing up from it either.
If you want to be careful then I'd chew up 1 and wait for it to hit me then chew up the other. And this is just to avoid taking too much that it makes you nauseous. Some people are more prone to it than others, depends, I for one rarely vomit and opiates never make me nauseous. But other people seem to throw up every time they get high. For some people its like a necessity to get high, they just know its coming and deal with it.
Simply put the answer is yes, it would be worthwhile if your goal is to get as high as possible off 20 mg of oxycontin. I assume you came her to get a question answered, not to be lectured on the dangers of addiction. So that is it, but obviously its a trade off, you can eat them whole and they will last around 8 hours and the high will be about 1/2 as intense. Or chew them up and its more intense but ends sooner.