I have done a little bit of researching - couldn't find anything in English as yet, but from what i gathered, oil pulling has been quite popular in Russia as a folk remedy. It originated somewhere around Caucasus mountains.
More recently (well, the 1980s) Dr N.T Karnauch from the Russian Academy of Science have done extensive research on oil pulling. I am not completely sure but i think he used sunflower oil or/and olive oil.
He argued that: oil need to be fresh, the fresher the better its haling properties. No explanation why (i couldn't find one), but he made comparisons between groups and concluded that fresher works best.
The important contribution, in my view, was that they looked and tested the oil in the lab conditions - before and after - and it was definitely full of bacteria that we don't want to swallow.
According to his research it takes only a few days to improve acute infections, but the chronic conditions take up to several months to improve. he noticed that with chronic conditions there is often a strong Herx effect, with physical and emotional symptoms. He warned against stopping when it happens - it is important to continue pulling even if there is seeming worsening of the symptoms.