The off-road type glowplugs are as we say of the hotter type which means that they will give an earlier combustion and so more low-end rpm power.

The on-road type glowplugs are colder which means that they will give a later combustion and so more high-end rpm power.

The hotter plugs can deal better with an engine which has a rich idle needle setting on the carburetor compared to a colder glowplug.

You can use whatever glowplug you like in off-road, however to avoid flame-outs(which is a challenge in off-road) you better choose hotter glowplugs. Also most of the clutches used in off-road come in early and so you need a responsive engine and for this the hotter glowplug is more suitable.

In any case you can always try a medium type of glowplug and see if it suits you.