Well Bosch said colder, NGK say colder, so that's 2 out of 3. Remember the combustion temps are hotter with LPG, so you need a cooler plug to dissipate the extra heat.
<b>The heat range numbers used by the various manufacturers are not universal</b>, by that we mean, a 10 heat range in Champion is not the same as a 10 heat range in NGK. Some manufacturers numbering systems are opposite the other, for American manufacturers (Champion, Autolite, Splitfire the higher the number, the <b>hotter</b> the plug. For Japanese manufacturers (NGK, Denso), the higher the number, the <b>colder</b> the plug.