Synthol is a sterile neutral oil that is locally injected right into the muscle. It becomes encapsulated in the bundles of muscle fibers which adds extreme volume to the muscl … e.The term Synthol used to describe any Site Enhancement Oil (SEO). Steroids are hormones.Source: Syntholmuscle.com.ynthol is a sterile neutral oil that is locally injected right into the muscle. It becomes encapsulated in the bundles of muscle fibers which adds extreme volume to the muscl … e. The term Synthol used to describe any Site Enhancement Oil (SEO). Steroids are hormones. Source: Syntholmuscle.com.