Leave in on... The only time you might want to disable 5th gear is if you were pulling a trailier up a long mountain pass and the car kept shifting up to 5th (loose RPM's) then shifts back to 4th over and over again... disable OD at that point to keep it in 4th.
I don't know why they didn't just put 5 gears (notches) on the shift column. That way you could just move the lever to 4 and that would be that.
I don't know why they didn't just put 5 gears (notches) on the shift column. That way you could just move the lever to 4 and that would be that.