A Site Enhancement Oil (SEO) is a liquid substance, usually a mixture of oils, used by some bodybuilders to increase the apparent size of some muscles. The effects of SEOs are purely and solely cosmetic and there is no increase in muscular performance.
SEOs work by filling the muscle up with an oil substance. It is site injected, that is, it is injected directly into the muscle whose size is to be increased. SEO is usually really only used to 'top up' a muscle that is not quite up to standard with the rest of the body, and once the use of SEO is stopped, all gains are kept, because the oil stretches the fascia and causes the muscle to fill the gap.
One of the more popular SEOs is Synthol, which was developed in the early 90s by the German bodybuilder Christopher T. Clark as a replacement for Esiclene which went out of production at the time. Synthol consists of 85 % medium-chain triglycerides, 7.5 % Lidocaine and 7.5 % Benzyl alcohol.