The lighter viscosity oils such as Citrus, Lavender and Ylang Ylang work best in a diffuser. Others of similar weight can be used too. However, carrier oils such as Sweet Almond oil are thicker, more viscous and are not meant to be used in a diffuser. If you would like to use a higher viscosity essential oil, make sure to dilute it with lower viscosity essential oils to create a blend that will be suitable for a diffuser.