ālū
ālū also has that –ū suffix, and it means 20 in base 8 or 16 in base 10. The stem āl is derived from an old word for 4, which is only ever found these days in derived words and in old poetry.
The other word for 20 is ēnnōr, and while it is rarely used in counting ōrū aþōnne (17), ālū (20), ālū aþān (21), it is used after aþ– as in ōrū aþēnnōr (100 and 20).