From all that I have seen, there are basically two steering wheel designs that McLaren fits to the F1.
The first is the standard Nardi 3-spoke design that the car has had since the concept phase. The rim and the center hub of the wheel are almost flush with each other and the spokes have gentle curves allowing for perfect hand placement. It has markings on it to identify the horn and flasher paddles located just behind the horizontal spokes as well. Some owners have chosen to have this wheel accented with various colors of leather or suede, but they are still the same wheel underneath.
Chassis #065
The other steering wheel is quite a bit different - more "sporty" if you will - and is rather close in design to what you would find in most GTRs and the LMs. It's not stamped with the Nardi logo, so it may not be sourced from them - more on this later. This wheel appears thicker in certain areas, and has notches cut into the horizontal spokes where they meet the rim for your thumbs to get a better grip. You can see that the center hub on this wheel is a bit recessed from the rim. It also lacks the markings for the paddles I previously mentioned. Again, different owners have had these covered in various material/colors for that unique look.
Chassis #073
For a while I thought that the second type of wheel was exclusive to the High Downforce Kit cars, however #038 (a standard road car) has it and I'm sure there are others that do as well. Then there are at least 4 HDF Kit cars - #020, #069, #072, and Paul Stewart's - that all seem to do without it. I was also under the assumption for a little while that this was a "removable" steering wheel, and the normal one was not, however the 'Juan Barazi/MAC IP' car (believed to be #049) has been seen with it's steering wheel removed, and it features the standard design.
The wheel in "Flemke's" car that was mentioned above is an OMP wheel that I believe he sourced and installed himself. It is similar, but not identical to the wheel you would find in most GTRs and the F1 LMs.
Here's a shot of the wheel in GTR #07R.
Note the vertical wheel rim is engraved with the word "Personal" - I suspect this may be the brand/manufacturer of this wheel, and the Type 2 F1 wheel above.
A quick scan of other GTRs shows them to have one which is nearly identical. Some of them have yellow stitching on the inside. Interior shots on GTR Lontails are hard to come by, but most of them have an identical wheel. I'm also pretty sure the "chopped-top" GTR wheel shown above by hurst is the style that JJ Lehto prefers, hence why it is seen in #01R. I did notice in a shot from the recent Goodwood FOS that #27R has a flat top and flat bottom design, but when it was at Laguna Seca as the Parabolica car in 1997 it had a "chopped-top" version.
Here's a shot from GTR #19R that shows the "Personal" logo a bit better:
Here's a shot of the wheel from LM3 - essentially the same as that GTR wheel, execpt the wheel center lets you know you are in a very special place. This wheel's vertical spoke is also stamped with the "Personal" logo.
The F1 GT longtail road cars - at least both #56XPGT and #58F1GT - have the standard Nardi road car wheel shown at the very top.
That about covers it... I hope.
>8^)
ER