My Warlock buddy (and my GM), told me about this bug today after I got home from work. Though it is unlikely your Succubus will ever go oom in a PVE encounter, it is still possible (tested, and after 160 seconds with no raid buffs, it oom'd).
If your Succubus ever becomes low on mana, or oom simply buff yourself with some sort of stat modifier (+ int procs, Kings, etc) and your Succubus will go up to 100% mana. This was tested IN combat, and it does indeed work.
Kinda hard to do screenshots of this, but Ill try if need be...