No matter what essential oils I use in my HP soap, the fragrance seems to diminish significantly within 2 months. 1) Does anyone have any thoughts on how to preserve the fragrance for a longer period of time? 2) I prefer using all natural ingredients - I'm not sure if any fragrance oils are considered natural? 3) Are some EOs more concentrated than others, and how can you tell? 4) What supplier(s) do you use for the best, most reasonably-priced EOs? Thanks! Barb R.