Plot no Jutsu. It doesn't really matter anyway. It's not like it's going to do much for his character. Give him a chapter and he will be forgotten yet again.
Nawl he only used Onmyoudon on people who he considered threats hint: MinatoThe technique he hit Sarutobi with didn't contain Onmyoudon. He didn't start infusing that technique with his jutsu until he gained complete control of his body.
Minato was hit with Onmyoudon, Hiruzen wasn't.He regenerated faster than minato , this was for the sake of plot -___- .