Mxmad's post is true. Let me expound a little more.
Modern fuel no longer contains lead. What the lead content in fuel did during its time was to lubricate the valves. However, since its prohibition some years ago, refineries have resorted to their own more eco-friendly additive substitutes. In turn, valve manufacturers have claimed to have toughened their valves. Basically, with these two changes both industries have come up with, I tend to think our valves are fine.
As for harm to the engine, mixing some small quantity of oil in the fuel (hopefully by mistake) wouldnt do any harm.
As for me, I just see myself not wanting (or needing) to do it.