YES - add oil !!! My baby is a medium sized baby. When he was 1 month old he was only taking 90-100ml each feed and 7 feeds a day. He then slowly improved and now at 2 months and 7 days old taking 120-145ml each feed 6 feeds per day.
So no worry, they will gain weight though may be slower than others. As long as they are healthy. 加油哦 !!!