The most common cause for this is the splitter in the transmission. Oil gets past the o-rings in the splitter cylinder and works it's way up to the shifter. This is usually caused by a slight pressure buildup at the splitter shaft when shifting which helps push oil past the o-rings. There is a fix for this which involves removing the splitter cylinder and drilling a hole through it about halfway down it's length. To verify that this is your issue just remove the little blue air line from the back of the transmission (lower left), if there's oil in it then this is the cause. If you wanted more proof then you could remove the air line that feeds air to the transmission's pressure limiting valve and see if there's oil there, it'll be on the upper right of the rear of the transmission, there'll be 4 air lines there, 2 run up the side of the transmission, 1 drops under the driveshaft to the splitter, and the other will run to the framerail, the last one is what your looking for. I suspect it'll be dry but your little blue air line will have oil in it.