I recently had a new

this could be 1 of 4 things

you have become use to change characteristics of the old well worn/broken down oil - but even so new oil should not be excessivey stiffer

incorrectly adjusted clutch - although 406s had a self adjuster cable

incorrectly installed clutch

or most probably

wrong gear box oil - i have experience this, garages will use one grade/type of gear oil - check what spec your car is meant to have and change it - it made a huge difference on my peugeot

the oil filler and drain are accessible by removing the road wheel on the gearbox side of the car