There has been a transmission belt part number change. I believe it is also 1/4" shorter. If the belt seems worn or glazed, it could be possible. Nothing keeping the clutch pedal from returning? More likely the transaxle. There is much discussion online on how to change oil on the transaxle.
I own a X300 John Deere and after mowing about 20 to 30 minutes it starts slipping, I don't know if it's the transmission or the rear end. Has anyone else had this problem, if so how did you fix the problem?
All X300-series tractors are hydrostatic. It sounds like the main drive belt is slipping. As others have stated, the transmission and rear end are all one unit referred to as the transaxle.