In Deamon Hunters there is a special astartes heavy flkamer called incinerator that uses special, blessed promethium.
It is a weapon, that a guardsman with a alternate career rank can learn to use despite it being an astartes weapon.

But as the belessed prom is not every day commodity, would you say it is possible to load it with normal prom if the ammo runs low? Or would it be too insulting to the holy machine spirit within?

Because changing to a normal astartes heavy flamer should be not option for two reasons:

• He would have to leave his holy incinerator behind (unless he wants to carry two heavy flamers with him)
• He doesn't have the ability to use other astartes weapons. Even other astartes heavy flamers that are just not as holy.

Or would you allow him to use other atartes flamers, too?

(I hope i remembered the weapon's name right. Only read it once a couple of days ago.)

Should be possible to use normal Promethium, too, but then it also should loose its holy-promethium-induced extra stuff.

the fuel is special as is the weapon. take out the fuel and its just a heavy flamer (thats normaly only used by space mariens)

so no, no problem using normal fuel you just lose the special damage. get your self a priest too bless it for you and hey presto blessed fuel again. the part is on again

yep, although the issue of insulting the machine spirit is important, make sure that the tech priest gives him an earful later when he goes to get it reblest, also you could give the gun a bit of personality, or even have it be able to speak to players via a tech priest..the fact that the guardsmen can use that is a little odd, but don't worry as long as it's justified let it go, not really worth fighting over.

The Astartes Incinerator uses Psy-Flame ammunition, which is described earlier in the book.

I most definately would not advise loading it with ordinary Promethium. The machinespirit would most likely rebel or, perhaps, become tainted... Depending on the GM.