Mike,
If you go to the Wacker website, you will see that the Wacker paste is made specifically for electronic heat sinks and not for spark plugs. While it is an excellent product and works well, it is not the only one out there. The radio Shack paste is made by NTF and is identical to the Wacker P12 in chemical content and thermal conductivity ratings.
Rotax probably uses it because it is readily available in Austria, whereas the Radio Shack product is readily available here.
Thet are both no more than zinc oxide with a small amount of silver, mixed in a silicon grease vehicle.
A couple of months ago, I posted under "spark plug paste" the temperature and conductivity ratings of each product as taken from their own websites, Wacker and NTF. You can probably search it out here somehow. (I'm not sure how to do it, I've never tried)
Bill.
