The Oath

The Gemorah Shevous (38b) derives from the fact that Avrohom made Eliezer take his oath while grabbing onto the milah that one must hold onto an object of a mitzvah when he takes an oath administered by beis din.  Tosfos asks why don't we learn from the oath of Eliezer that every oath requires one to hold an object of a mitzvah? The Rosh suggests an answer that the limud from Eliezer is just an asmoctah, the din is only rabbinic to scare a person.  The Ramban says this same idea even assuming the din is a Torah requirement.  See Torah Temimah for a different approach.  Interesting as well to note his challenge on the Rosh that what basis does he have to say the derash is merely an asmochtah.  It would seem since Tosfos didn't give this answer that he holds the grasping of a mitzvah object isn't to scare the person, its a chomer in the oath.  An oath that is taken while holding an object of a mitzvah is more chomor than an oath taken without holding a mitzvah object.  
The Gemorah says that the oath should be administered with a sefer torah but for a talmud chacham its enough for him to swear holding tefillin.  Why is a talmud chacham different?  If the reason of holding a mitzvah object is to scare the one swearing,  its understandable for the talmud chacham understands the severity even when just holding tefillin.  It could be even Tosfos agrees that which object is held is measured by what will scare the person.  The basic idea of holding a mitzvah object is to make the oath more stringent, but the exact object is decided by "scare points." See Tosfos as to how we derive a sefer torah if Eliezer was holding milah, ועדיין צ"ע.
The Gemorah says that is the oath is administered without the plaintiff holding a object of mitzvah that it is considered making a mistake about something that is in a mishna.  According to Tosfos the din is understandable for the oath wasn't a good oath in this scenario.  However, according to the Rosh why should the oath not be good, he took a proper oath?  The Rishonim say that we also light candles and blow them out to scare the plaintiff, if those are lacking we don't have to redo the oath?  The Meiri that answers this question that we are afraid the person feels without holding an object of mitzvah the oath is only rabbinic and that he is willing to violate, therefore we make him redo it where he knows its a Torah oath to see if he will retract.