An Ashkenazi woman discovered a Pesach recipe that belonged to her grandmother. One ingredient is peanut oil. Is she entitled to return to that prior Minhag of her family?
See Rav Moshe (Iggrot Moshe OC 3:63), who allows peanuts for those that don’t have a practice otherwise. He specifically mentions at the end that he has seen that they give a kosher-li-Pesach hekhsher on peanut oil. OU states that only “some” have the practice to not use peanut oil, see here.
I would say that unless she was consciously adopting a Minhag to not use peanut oil then she can rely on all those poskim that permit, and does not need heter nedarim. If she consciously adopted not using peanut oil as a practice, she would probably need heter nedarim. If this is what she was doing because her rabbi told her so (or some Pesach guide told her so), that is not consciously adopting as a practice, and she can use peanut oil without doing heter nedarim.