Any matter admitted under Rule 36 is conclusively established unless the court on motion permits withdrawal or amendment of the admission. Subject to the provisions of Rule 16 governing amendment of a pretrial order, the court may permit withdrawal or amendment when the presentation of the merits of the action will be subserved thereby and the party who obtained the admission fails to satisfy the court that withdrawal or amendment will prejudice him in maintaining his action or defense on the merits. An admission made by a party under Rule 36 is for the purpose of the pending action only and is not an admission by him for any other purpose nor may it be used against him in any other proceeding.