It isn't... not on the older 2.0 ones anyway, which is what the 306 will be if you get one.
I've had several HDIs... they're very sensitive economy wise if
*anything* is amiss under the bonnet.If possible, get a 110bhp rather than a 90, as weirdly, they're both more torquey and more economical overall when both are running right, although the 110 isn't found in the 306.
****The 110 is intercooled, so it's more efficient...
IME, the XUD has less bits that fail under the bonnet than the HDI.
The HDI is quieter and more economical when running right.
The XUD (TD at least), can be tuned with a screwdriver, the HDI needs a map - both go notably better once tweaked, and both will cover high mileages if you look after them... although the HDI will present more problems without obvious answers, most of which will cause it to lose power and use more fuel, along the way. :-)