On my '98 it is a T-30 torx. You need to have an extended length Torx; mine are 6" long. They need to be at least 4" long, and 6" is about as long as you can go and still clear the dash. I bought a set of Gear Wrench brand from Amazon fairly cheap - 10 bits from T-8 to T-50.
The steering wheel bolt itself is quite hard to remove on some of these. Of course, they use threadlock... I find it best to soften the threadlock with some nail polish remover (be careful not to get it anywhere else), and use an impact wrench for the steering wheel bolt removal.
Some tough cases also require careful use of a heat gun. The worst one, I've had to drill out.
Just thought I'd add to this NOT to lock the steering wheel when you are trying to break loose the bolt, or you might jam the steering lock. Hold the steering wheel with your hand while applying torque to the bolt. See the following, excellent thread on what happens if you torture your steering lock, and how to fix it: http://www.benzworld.org/forums/r129-sl-class/1592845-ignition-switch-lock-1998-sl.html. Great work, patfarrell, aam, and WarrenC!
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