Node View Truth Value Not Working Properley

When i create a rule on the event "content is going to be viewed"
and set the condition to "check a truth value" and set the that value to "node:nid == 20"
and set a action to " grant user role".

The rule is activated on any node view.
any node the user visits the rule is activated and the user is granted that role.

i tired changing the condition value to " $node->nid == 20 ", now any node viewed even node 20 doesn't activate the rule, and the user isn't granted a role.

Someone please help