There are two key pieces of equipment that have escaped the conversation so far which allow for unlimited inverted flight, even with a wet sump engine.
The first is a flop tube. This is just a piece of plastic tubing with a sieve on the end which is mounted inside the tank. It flops around inside according to the g-loading on the aircraft, thereby always staying inside the fuel, wherever it is in the tank.
The second is the Christen inverted oil system. It's mounted to wet sump engines and consists of tubes that pick up oil from both the sump at the bottom of the engine and the breather at the top. When the engine is upright, the oil is drawn from the sump. When the engine is inverted, the oil collects in the upper part of the engine and is therefore drawn from the breather. The inverted system is a seperately mounted cannister which has some tube and ball bearing contraption inside which I've never fully understood but keeps the oil coming and going properly and keeps the air coming and going properly (from the breather upright, sump inverted). Don't ask me exactly how it does this but it works.
My Pitts has both (as do most but not all Pitts, as far as I know) and can fly inverted all day long (or until said flop tube can no longer connect with any fuel because there isn't any.)
Pitts2112