According to Jaro's document on bow woods:
Lilac (Syringa vulgaris, Syringa L.)
SG 0.7-1.0
Tension 8
Compression 9
Response ? (Unknown, but higher)
It has white or beige coloured wood, which is very hard and has good mechanical properties. It is very difficult to season, and it can crack in matter of mere minutes. However longbows of approx. 50# have been made of it, and those have reportedly shot very well.