I've seen some shampoo advertised as residue free that contains Hemp seed Oil, Peppermint, Green Tea, Lemon-grass etc. How could it be residue free? - Dreadlocks FAQ's
The short answer is: If it contains those ingredients it's not residue free. If it were, it would mean that all of those things that it contains rinse out completely during each washing. If that were the case why would they be added? The way your hair would smell and feel after you used that shampoo and tried to rinse it away would make it clear that it does not rinse out. If it doesn't rinse out completely it's not residue free.
The company advertising this is either dishonest or ignorant (possibly both). They've added a bunch of ingredients that have a perceived benefit if left in your dreads. Then they tacked on the term residue free without any understanding of what it means. (If they do understand they're betting that you don't) As people become aware of the benefits of residue free dread soaps I would guess you'll see a lot more "me too" claims like this.
I've even seen the term residue free used for home made bar soap and Castile soaps! These soap are made from vegetable oils or animal fats and either way their basic cleaning process results in precipitates that do not get washed away. These residues or precipitates are a by product of the way that type of soap works and they are left even if there are no extra additives or fragrances in the soap. These soaps are fine for most applications where residue has no consequence, except maybe a ring of soap scum in your sink or bathtub, but you'll never see this type of soap used in a setting where clean is important, like a doctors office or an operating room.
When you're first starting your dreads you want the hair to be as clean as possible. Any residue that reduces friction is working against you. Later, after your dreads have matured, friction becomes less of an issue. As long that the new growth continues to knot as the hair grows the friction is sufficient. At this point the concern with residue is that if residues are allowed to build up over time they will slow drying and can lead to dread rot. For this reason you'll want to continue to wash your dreads in a residue free soap even after they've matured, unless you have solid, workable plan for regularly removing residue that other soaps and shampoos leave behind.