5 speed tranny

I own a 97 M3 w/ manual tranny w/ 42000 miles. When the car is cold it grinds on 3rd gear. I was told to change the tranny fluid and it requires an ATF (automatic transmission fluid) oil. To my old experience, I had an Accord 5 speed (Honda) that grinds the same as my BMW. When I check the tranny fluid, the previous owner had red fluid in it and assumed that it was ATF, I change it to gear oil and behold the problem is gone. With this history, I opt for synthetic gear oil instead of its original ATF. Now the there is a little grind on 2nd and not the 3rd, but when giving a little gas before shifting it goes in right away or in normal driving condition.

But my main problem is when driving a little aggressively. When downshifting from 4th to 3rd in a hi rpm, it wont engage and grind totally or when rolling in neutral and engaging in 3rd.