A friend has a 1999 mazda 626 automatic it will shift but not go into gear holds fluids but her bro may have put oil in the trans fluid would that effect it it will try to go into gear but not when car warms up or could it be the cable or both what should I do? I believe it started not going into gear before the idiot may have put the wrong fluid in also isn't trans fluid red because it is black now please help for I may purchase the car if it is not to far off ty

Yeah... motor oil can and probibly has ruined the transmission. Depending on how long it was operated like that.

You will have to drain it and change the filter several times because the torque converter holds fluid that will be difficult to drain properly. At that point it MIGHT work...or it might not. Since clean motor oil is not black....I'm assuming the clutch packs are burned up.

I would think twice even if it was given to me for free... replacing the transmission is neither easy or cheap. I would not pay money for it in this condition.

How can it "shift" but "not go into gear"?

I'm guessing its "not going into gear" because the clutch packs are completely gone...