Drop the pan, check the pump. Sounds to me like it may have been an intermittant problem either sucking or pushing oil, then finally failed altogether. I always say check the cheep solutions first. Look at the pump and pick-up, replace if nessesary, then check again. If that doesn't do it, pull the whole thing out and tear into it.
68 fastback, non-GT S-code, Just a little longer and it'll be done!
Just added a 71 Mach to the list.