300 000 iu im injection of d2 raised vitamin d levels from 8 mg/ml to 20. What oral dose of D3 would you recommend now and why? How long for?

5-10,000 IU Daily. I think you mean ng/ml. I like my patients to be in the vicinity of 70 ng/ml. That means you would need to increase your levels by nearly 60 ng/ml from where you are now. I suggest 10, 000 iu per day of d3 for at least one year. You could re-test at any time to see how you are doing.
Varied dose . I think your level should be between 50-60. You would need to take oral dose of 10000 iu daily for at least one month and then decrease to 6000 iu for a month and then should be able to get to a maintanence dose of 2000-4000 daily . You don't really absorb much until the level is around 30.
Vitamindcouncil,org. 1000 iu / 25# body wt long term/lifetime i do not like or use d2 any longer want a d level>50 d3 is better than d2.