​​So what are the main sources of unpure bloodlines within the pure bred Arabians? 

Although very famous, the Polish Arabians are not asil as several of their foundation mares are undocumented and are probably tracing to Polish local mares and not desertbred Arabian stock. One of the most famous Polish Arabians was Skowronek who was very influential in Crabbet Arabian horses.

Although Skowronek’s sire, Ibrahim, was desert-bred and of pure Arab stock, his dam, Jaskółka, came from Polish-bred lines that included a mix of non-desert and even Thoroughbred ancestry. Genetic analysis revealed Skowronek carried genes from roughly 13 Polish regional mares, 5 English Thoroughbreds, and one Turkmenian ancestor. Through Crabbet, the blood of Skowronek spread to the majority of the Arabian bloodstock in the 20th century.

Another important source of impure bloodlines was the stud of Ayerza in Argentina. Although Hernan and Alfonso Ayerza ran a stud with some of the finest Arabian bloodstock in the early 20th century, they had imported Shagya Arabians descending from 30 Maria like O’Bajan VII and Kurdo III and mistakenly took "Araber rasse" on their papers as pure Arabian while it is a German designation for Arabian related. This blood spread early on to Spain where it ended up in the Veragua mares and in the late 20th century, WAHO made the contraversial decision to recognize the horses in the South-American studbooks with the blood of 30 Maria as pure bred Arabians.

In France some of the famous racing arabs of such dubious orgine were Manganate, Saint-Laurent and more recently Amer. The Anglo-Arabian conformation of these horses speak for themselves.

Today, less than 5% of the total Arabian horse population is considered asil. The largest breeding group of Asil Arabian horses are the straight Egyptian Arabians. In the Middle-East important number of Asil Arabian horses remain with the bedouins in Syria and at the Royal Stables of Bahrein. The very large asil breeding group that was remaining in Tunesia until the 2nd half of the 20th century has mostly been destroyed by using French racing stallions. In the United States preservationist breeders are still actively breeding with asil Arabians like the so-called Davenport Arabians as well as direct imports gifted by Saud. The status of the Turkisch Arabians is not clear yet. A large population of their breeding stock still descends entirely from desert stock and might still be asil, although some suspect that one of 4 foundation mares coming from the sultan's stables migth have had Polish blood.

So all Asil Arabians are purebred, but not all purebreds are Asil. As such the Asil Arabian is not necessarily a stronger performance horse or more typical Arabian but as described in literature going back to the late 18th and early 18th century the pure bred Arabian will always needs infusion of Asil Arabians to keep the breed's dominating characteristics.