[Funnyman]

2.4 quart of oil inside the transmission is way too much.
The problem come from the inside of the transmission. Not from the clutch.
Toyota C series 6 speed transmission : 2 quarts for LSD equipped - 2.2 for open differential.

Too much oil inside the trans > too much pressure inside the trans.
Cause the gears/syncros/bearings/everything inside the gearbox to work too hard.
That's why you have difficults to change gears. To be quick : too much oil will kill your transmission.
Do not drive and change gear at high rpms. This will shorten life of the internal of the transmission.

Drain all oil again, then refil with the correct quantity : 2.2 quart or 2 quart.
To give help : when refilling the gearbox with the correct oil quantity, oil should not be level to the “speedo cap”.
If it's the case > the car is not on flat road, the back of the car is lower than the front and/or the oil has not been fully drained.

 

[Funnyman]

Is it really possible for excess oil to prevent gear shifting in a manual transmission?? I’ve heard of automatic transmissions being sensitive to oil level since they shift on hydraulic pressures, but a manual transmission only has gears sliding on shafts, and synchro rings made of brass engaging a tapered steel surfaces. I can’t see how excess oil could hurt anything but possibly the rubber shaft seals (due to elevated gear case pressure?)
I have read that some manual transmissions will baulk at high rpm shifts. I know my S51 transmission occasionally doesn’t like to shift at high rpm. But what’s the point of a high revving 2zz engine if you can’t shift gears, at high rpm? I believe there are hundreds of 2zz engines running with C60 transmissions. And I doubt they all shift gears at low rpm’s.

Yes it's possible (Toyota C 6 speed series does not have breather valve). I did have made this mistake. My previous transmission was fine before installing it. But after over-refilling the gearbox, I started to have more difficulties to shift gear. I tought it was because of winter season. The trans has started to make whine sound after 6 months of overfilling and 2nd/3rd synchros have started to failed. When pignons/diffferentials/ect are spining quickly while splashing oil, it create high pressure. And no need to go fast to create high pressure. From the video below, imagine if the transmission was fully oil filled.