Grinding when shifting -sometimes-

This may be a shot in the dark but what type of fluid is in the transmission if the fluid is thick and stiff even though the clutch pedal is on the floor it takes time for the internal gears to stop turning so it grinds when going into gear. In a pro-shifted transmission I use 5W20 motor oil which is always thin but still slippery however I need a new set of gears after 70-75 passes just to be sure nothing breaks.