Adding lemon oil to dish soap makes it so much better, in my opinion—and it makes the soap stay smelling fresher longer. It helps the dishes get squeaky clean, and smell/feel good.
IMO (or according to my nose), dish soap goes bad when exposed to certain things long enough (probably mold, yeast or bacteria), but adding lemon oil seems to prevent this kind of spoilage from happening after opening the bottle (never had a freshly opened bottle be bad). I say it goes bad since it smells a certain kind of bad and makes me feel sick if I use it.
I especially liked adding it to Ajax lemon dish soap, but it works with everything I've tried it in.
As a side note, my new favorite dish soap is perhaps Ultra Palmolive Pure+Clear. Either that or Equate Baby Wash & Shampoo (which isn't advertised as dishsoap, but it works well for it without making dishes slimy). I haven't tried lemon oil in the former (nor had the need), but I have tried it in the baby wash (for using with dishes).
I have chemical sensitivities; so, that's why I use particular dish soaps.
Dawn doesn't seem to be good for me, even fresh. There must be some chemicals in it that don't jive well with me, whether or not baby commercial ducks like it.