Even the VII hierarchy says it's not a sin.
Excerpt from letter of the Pontifical Commission Ecclesia Dei, January 18, 2003
His second question was "Is it a sin for me to attend a Pius X Mass" and we responded stating:
"2. We have already told you that we cannot recommend your attendance at such a Mass and have explained the reason why. If your primary reason for attending were to manifest your desire to separate yourself from communion with the Roman Pontiff and those in communion with him, it would be a sin. If your intention is simply to participate in a Mass according to the 1962 Missal for the sake of devotion, this would not be a sin."