You might want to consider "mistruth."
http://chuckcowdery.blogspot.fr/2012/01/mistruth-in-alcoholic-beverage.html
https://norrie.wordpress.com/2009/01/16/
http://www.becomingminimalist.com/superbowl-ads-mistruth/
E.g.
If it's not a lie, but a mistruth, is it slightly less false?
An opinion can most certainly be a mistruth when presented as a fact.