One has to make a distinction when referring to Romania since it was different for areas which became part of  modern Romania in 1918 — Transylvania, Banat, Maramures, Bucovina all part of Austria Hungary —  or Bessarabia - part of Russia. For the Old Kingdom, the most relevant is Moldova since most Ashkenazi Jews living in Wallachia (Bucuresti) , may have originally arrived from  Moldova.  The vast majority of the Jews came to Moldova in early mid 1800 from former Poland lands, primarily Galicia (then Austria), Podolia (then Russia), Bukovina and Bessarabia. As such, they must have had already surnames from the country of origin. While surnames were required in Moldova/Romania since 1860s, since Jews were not  citizens, those laws were practically not enforced. Therefore I see a phenomenon where the original surnames may not have been used by all, possibly, most Jews. Instead there are many occupational names (Romanian) and patronymics (X sin Y). There is a variability of names found in civil records, e.g. a tailor may be referred as “Croitoru” and later as “Nadler” and who knows what the original Ashkenazi name was. A money changer may have been named (a turkish origin name)  “Zarafu” and his children became “Vecsler”. For Jews which kept their Austrian or Russian received names, A Beider reference books are the best resource. But is may be unknowable, whether someone’s surname in the 20th century reflect an earlier surname or one acquired later.

Luc Radu
Great Neck, NY