He's not removing the steering wheel. But the clamp that holds the assmbly into the steering column.

And yes its 10mm. None of the ones I've messed with have been on all that tight. Perhaps mine have been messed with in the past.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.