People pull up gunk all the time from old oil. No big deal.
How long can it be unabsorbed for? That depends on a lot of factors, so can't really put a time frame on.
How many cc's do you inject with?
Try rotating injection sites better.
Still keep an eye on it to see it is not an infection. I wouldn't worry or dwell over it because I would bet to say it's just old oil.