Dimensional,
The aircraft that I fly can sustain inverted flight for an unlimited length of time due to having the oil system that Pitts2112 describes above. When it is inverted with constant -g applied the oil system functions quite happily, although sometimes with slightly reduced pressure.
The aircraft is, however, limited to 10 secs of zero g flight as you mention due to the oil and ball-bearing valve being neither in the 'top or bottom' of the engine.
ST