Warning: Spoilers allowed in answers
Ever since The Eye of the World prologue, Ishamael/Moridin seems to channel the True Power exclusively.
We know that Elan Morin Tedronai was considered Lews Therin's equal in the One Power. But I'm working on a theory for why
Rand couldn't channel Saidin after 'switching bodies' with Moridin.
So, do we ever see Elan Morin, either as Ishamael or Moridin, actually wield Saidin?
Just got back from a vacation to the US. I was lucky enough to be in San Diego when Brandon Sanderson was signing, so I asked him. He "MAFO'd" this specific question. So I asked if Moridin was in fact no able to channel Saidin. He responded with a grin and, "That's a very good question." I think this might be the new form of RAFO.
So, if we could find an instance of Moridin channeling Saidin, then this question can be put to bed. If not, it might suggest that he couldn't, but it's not proof.