Having the right deep conditioner can make your hair soft and silky. However, this conditioner is the one that you choose as per the prevalent condition, type, and texture of your hair. Otherwise, the chosen conditioner can give dull and oily strands.
Not all deep conditioners are same, as they can a variety of penetration levels. For example, the Phyto brand itself offers deep conditioners, of which one is less moisturizing than the other. Similarly, some conditioners give more lasting results than others. Generally, deep conditioners have proteins, amino acids, panthenol or a mix of all to fortify and soften damaged or chemically treated manes.
In case of doubt, consulting an expert is the way to go.