Some do, some don't. Some of what looks like SEO is from site injecting gear. At higher levels of competition, some guys are using massive amounts of gear, and it has to go somewhere. biceps, delts, ect. SEO or oil based steroid injects will create local inflammation and swelling. This is not real muscle growth though and I'm not a big believer that stretching the fascia with oil will make a difference in muscle growth. In general, I think that using oil to increase size can be very risky. Not only due to the chances of infection, but also it often times doesn't look right. You'll be more successful with small muscles like delts or biceps. But even when the user does a good job with it,  the muscle only looks decent when it is actually in a pose. Between poses, when the guy isn't flexing and has biceps that still look pointed/peaked, it just does't look right. You can tell something is off. In most cases, muscles tend to get distorted with higher volumes of oil and this looks fishy. Big muscles like quads, take on a lumpy look when guys try to "enhance" them.