Some people on a budget have achieved decent effects with the likes of peanut butter or cooking oil, then washing the rest off.
But if you have access to industrial level chemicals, then use any of the above mentioned products. Some people have used WD40 to great effect (which to be honest, with metal, might be a good combo).
I noticed another guy on youtube using a combination of goo gone and a penny to get some abrasive action going. Worked pretty well.
I personally use window cleaner, let it soak for 10 - 20 minutes and it usually just wipes right off. If that doesn't work, then I use the same process but using sunflower oil instead. I haven't found anything that either one hasn't worked on, non abrasive too!