These Citroen engine oil level gauges has been more a concern than usefull info to their owners.
They are there as a meaning of go/nogo for open the hud and do an engine oil dipstick level control - nothing else.
As years go by - the harsh environment round the engine and from the road - makes this level control sensor & cabling unreliable.
If you want to bother - the level sensor is a long odd dipstick looking thingy with 2 wires attached on top - located on the rear of engine down in the sump. It's very uncomfortable to reach - and many earlier models simply had the wires cutted by garages - to stop worry owners.
As for the stiff gearshifting - check the clutch pedal height level - should be resting approx 145mm above carpet/mat level - no matter any extra mats/carpets used - i.e. the actaul height above any floor covering.
Might do the box good to have the oil replaced - which is also the only way to determine if the oil level is correct.
Use ONLY 75w/80w API spec GL4 gear oil - never a synthetic - never a higher grade API spech GL5.
Available from f.i. Total as BV gearoil (which simply means gearoil for manual transmision boxes).
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C U CX22TRS '86 - BX19RD '89 - BX16TRS '89