I have emailed ZDZ directly and they told me that they do not recommend premium gas. The octane rating they recommend in their manual is based on the European method of rating octane....which I think they use the research octane number. In the US we use an average of the research octane number (RON) and the motor octane number (MON) so when its all said and done in the US you only need 87 octane for the ZDZ's...I imagine the RCS is the same way.