Is this the right picture

Yep that's it

Looks more like scale than sooty mold.

You have scale. Very common on all types of citrus. The sooty mold is just a by-product of the insect infestation - scale is a sucking insect and excretes a sticky substance called 'honeydew'. Sooty mold is a fungus that grows on the honeydew. Of itself, sooty mold is not a serious problem - it really doesn't harm the plant unless it becomes so thick on the foliage it prevents photosynthesis.

Neem oil is a pretty effective scale control but you will need multiple applications. Ditto most horticultural oils. Neem can also prevent the formation of sooty mold but will not remove it. You need hand washing to accomplish that.

Here is a link that might be useful: controlling citrus scale

Thanks, that's what I thought but the absence of results using white oil made me worry. I'll try using neem oil as you said and see what happens! Thanks for the link!

This post was edited by ProfDeusc on Sun, Oct 12, 14 at 14:29

They won't disappear until they're ready to fall off or you scrape them off.

The oil or whatever else you use targets the youngsters. So it's best to treat when they're present. For many scale, that's May or so. Once you determine which scale it is, you will know when the eggs hatch and when is the best time to spray.