How can I set the view mode of a node through rules
What "Content is viewed" actually means, seems to be like so:


The event Content is viewed is what it is: the user has (already) started viewing the content.
The (system) event Drupal is initializing is what you should use instead (possible combined with other Events and/or Actions), if you want to trigger some Rules "Action(s)" BEFORE the user can start viewing (= looking at) the content.


For more details, and a working sample (in Rules 'export format'), refer to
How to specify a Rules event like &quot;Content is &#39;going to be&#39; viewed&quot;?

Be aware: this applies to D7 ... and D6 is another story (from the question context I'm not sure if this is fopr D6 or D7 ...).