I don't know what the virtual minimum oil level is but I haven't yet run out of oil before I've run out of gas.
If you mean what was the historical minimum quantity, this is an interesting question but one that could probably only be answered by an engine mechanic -- and the 'minimum' amount of oil needed by the engine would only be theoretical IMO.
All of the vintage Pilot's Flight Operating Notes and training materials in my collection give roughly the same limited information about the P-47D's oil system: the tank has a 28 gallon capacity but that 19 gallons is the normal level and the rest is overflow. The oil level could be checked on the ground by opening a petcock on the left side of the tank and near the filling spout; if oil dripped out, the aircraft had at least 19 gallons. (The A2A WoP3 P-47D's loadout menu seemingly assumes that 19 gallons is the proper amount and that is the maximum quantity for the model). Thus, it seems that the normal pre-flight check included ensuring that the aircraft had at least
19 U.S. gallons of oil on board before start up, and possibly extra oil up to 28 gallons.
As you may have noticed, the cockpit does not include a gage that measures amount of oil in the system; instead the pilot is only able to monitor oil temperature and PSI. This is very likely the reason why the operating notes and training materials do not provide a minimum quantity of oil needed to operate the engine; the pilot wouldn't be able to know while flying when he was nearing that dangerous quantity anyhow. Instead, the operating instructions explain to watch the PSI:If the oil pressure doesn't come up within 30 seconds, stop the engine at once. Lack of pressure can destroy and engine in an incredibly short time. (On a cold engine, the oil pressure clibbs quickly to 200 psi.) Keep the engine below 1000 rpm until the oil temperature is above 40 degrees C, and until the oil pressure is below 100 psi.
(AAF 50-5, March 1945, p.45)The oil tank pendulum allows 7 seconds of inverted flight. When the pendulum is operating properly, oil pressure drops, then rises to 50 psi as the plane completes a maneuver involving inverted flight. If this doesn't happen, the tank is defective.
(AAF 50-5, March 1945, p.23)Oil pressures should be as follows:
Desired..............75 to 85 PSI
(Pilot training notes, c.1944)
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