The rule of thumb is that if the Spirit leads you to use it, then use it. And if the Spirit hasn't led you , or you haven't consulted him about it, what use is it? The problem is you want to make it a rule, so that you can say, if I use oil, then God must heal the person, it's holy oil. But that's according to legalism and religion - God doesn't work like that. Everytime Jesus healed someone he used something different, sometimes mud, sometimes spit, sometimes nothing at all, sometimes hands, sometimes a spoken word, sometimes the hem of his garment. See the picture? So put away that little religious thought that you must do something to get something from God, and start listening to what the Spirit will tell you to do in that moment. That is assuming that you have the Spirit, and you do if you are a believer, and that you haven't quenched and grieved Him so that He remains silent..
His sheep hear and know His voice, so if you don't hear and know His voice, are you a sheep? And if the Spirit has led another to use oil, but not you, then who are you to judge? are you his master? Consider your motives for questioning your brother's actions.
Use of holy oil is not a condition of God answering the prayer of a righteous man. But if it helps build faith in the person using it or the person receiving healing there's nothing wrong with using it I don't think.
An elder or pastor who cannot heal may not meet the condition God requires i.e. no sin or iniquity in the heart, lack of faith, etc. Or the person receiving the healing may lack faith - it's not just about the pastor/elder, the recipient needs faith too.
If we were strong in faith we would only say a word from a distance and people could be healed. Most of us are weak because we have regular jobs and stresses and pressures of life from family, friends etc, so don't have the time or will to cultivate the faith required, so we need props like oil, prayer cloth, music, bible etc to bring us to that level of faith so we can see people healed.