I used the SEO (Site Enhancement Oil) from Nuclear Nutrition (Trevor Smith) back in the day. That was a much cheaper alternative to the name brand Synthol which was crazy high back then (15 years ago).
Synthol use was kept pretty quite at the time. Steroids were cool, Prohormones were gay, PCT was an extra and Synthol was cheating.
I mainly used it on my biceps. It works well in smaller, more rounded muscles. It would also work in the delts. I tried it in my tris and didn't like it. It was rather painful (afterwards) and I was never quit sure where in the tricep to inject. You want to get close to the center of the muscle belly for proportion.
This was my argument for Synthol back then and I'd have to say that I still feel the same. Synthol helps the muscle grow by stretching the muscle, thus lessening the elasticity. Similar to the benefits of muscle memory. Also similar to a good pump. If you temporarily stretch the biceps from 18.5" to 19" each week, eventually you reach 19".
My muscles (biceps) never returned to their original size. Simply work out hard, eat right and slowly grow into your newly stretched muscles.
It also hurts like a bitch when you work out. Your muscles have never felt so stretched.