Q: Stiff gearbox, difficulty when shifting gears

I bought my manual transmission,6-speed '08 Hyundai Tiburon around 8/15/16 with about 91,500 miles on it.There was only some difficulty in shifting gears when I test drove the vehicle. Between 9/1/16 and 10/10/16, I've found it is extremely difficult to shift gears- it's the most difficult when putting the vehicle into 1st and reverse. Around 9/6/16,I had a reputable local mechanic drain the old gear out of the vehicle and put fresh gear oil into the transmission case.The old gear oil was a very dark black in color.The gear oil change didn't seem to help with the tough shifting. Sometimes, double-clutching makes it easier to shift gears.The dealer I bought the car from said he had his guy put it in a new "flywheel" and a new "spindle disc." The slave cylinder looks new to me too. I found the original owner/1st owner of the car and he said he sold the car around 12/1/15 because "the clutch was going bad." When the car is off, and I try to shift gears, the car shifts like butter.

My car has 95750 miles.
My car has a manual transmission.