96 m5r1 reverse troubles

[h_2_o]

OK my dad has a 96 ford ranger 4 cyl, 2wd, with a mazda m5r1 manual trans in it. he took it to a local mechanic and it was a bit low on fluid. they put 75/90 in it. I realized something was wrong and that was the incorrect fluid. less than 200 miles were put on it between that time and a drain and refill with mercon atf (ford dealer told me to put that in). so i drained it completely, actually let it sit draining while i went to get the fluid ~ an hour. put it back together filled it up correctly with the correct fluid. now comes the problem, when going into reverse, about half the time it grinds horribly. the rest of the time it goes into gear normally. Since that time as well shifting has become a bit harder too, not horrible bad, just not normal.

so what can be done about reverse or does it just need used in reverse to get any heavier oils out of there that should not have been there to begin with?

and shifting is there anything that can be done about that or is this one of those get the fluid hot on a good drive and do another change deals and it might take a few changes?

thanks for any and all suggestions.