Why not?

 

According to Rogue Engineering (this was regarding BMW but it applies to all manual transmissions):


Shift **** No-Nos

Why can't I rest my hand on the shift ****?

Ever wonder why people say that its bad to rest you hand on the shift **** while driving? Well, there is a reason, and good thing you listened!

Within the transmission itself, BMW has a specification of 0.004"-0.006" gap between each gear and the shifter rail in the transmission (for reference, a human hair is approximately 0.0035"). This gap allows for the transmission lubricant to acts as a barrier so that the shifter rail and gear are not directly touching, causing premature wearing of the gear (not syncros), shifter rail, or both.

When you make a 0.001" change at the shift **** and maintain pressure (like resting your hand on the shift **** while driving), the required 0.004"-0.006" gap is closed, preventing the lubricant to ride between the gear and shifter rail. Premature wear of the gear or shifter rail may hamper transmission shifting performance, gear engagement, or worse, require rebuilding or replacement of the transmission.