On Augustus' Actual Prescription to Restore the Julian Calendar to Accuracy
8 AD was definitely a leap year.
It was believed since Scaliger that the leap year sequence was (45), 42, 39 ... 12, 9 BC, AD 8, 12 ... This was based strictly on sources in literature. Chris Bennett claims that an astronomical papyrus published in 1999 (pOxy 61.4175) which gives lunar ephemeris in late 24 BC and implies that Scaliger was definitely wrong, and the correct sequence is actually 44, 41, ... 11, 8 BC, AD 4, 8, 12 ...
The major source for Roman chronology questions is A. K. Michels, The Calendar of the Roman Republic (Princeton, 1967).
If you just want the answers, Bennett's page has clear tables. It also has a very good list of primary sources.