I'm sure I'm opening a can of worms, but I was reading an article (probably Pat Arnold) which stated the most internesting thing, along the lines of "past the dosage required to show gains, the results from increasing dosages are logrithmic".
Put in layman terms, once you get up to the dosage needed to see results (probably somewhere equal to your normal test levels) then for everytime you double the dosage you will only see 50% more gains.
So, with 80mg/day of ox one should expect to see only 50% more results then with 40mg/day. (And also that 80mg/day will only give you double the results of 20mg/day.)