has it always had this or has it just developed recently? could it be sunburn?

Some of the plants I have in stock do this, and I think it's where they get dry. Although lilacs love a sunny position, and well-drained soil, they suffer if they can't get enough moisture at the roots, and I think that's possibly what's happened to yours. They also prefer a chalky soil, which is true for all of them

Looks like it is trying to emulate its hosta neighbours, Lol.
There is a variegated lilac, could it be that it had the wrong label on it?
I have found this website quite interesting: http://www.buzzle.com/articles/types-of-lilac-bushes.html maybe it will help?

Thanks everyone! I think Worthy must be right. It was near my Acer, which took all the moisture from the soil, so perhaps now the Acer is gone, it might be happier. I'll feed it a bit of Lime too, thanks Worthy!

I think your lilac, (Syringa) is in fact a variegated variety. The green leaves are a reversion and will eventually dominate the plant. If you are happy to have a green variety then leave it alone, if not compost it.

Jimmy, that's an interesting remark, and I had wondered about that, so before I asked the question, I looked for a variegated var. on the web, but couldn't find a variegated dwarf syringa anywhere. I suppose I could ask at the nursery where I rescued it, perhaps they will remember.

It is my belief that variegated varieties of most plants begin life as 'sports' being a freak of nature. I also, have not seen a variegated syringa so you might have something unusual if you can take cuttings of the variegated stems.

You might have seen this one Jimmy...Syringa emodi variegated...I have it! It's lovely.

http://www.growsonyou.com/photo/slideshow/247821-syringa-emodi-variegata/member/karensusan63