I cannot convince a non-aircrew colleague of mine that on large supercharged piston engines with constant-speed-propellers, if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm with accompanying boost (manifold pressure) and oil pressure. The only indication of the engine failure -if you missed the initial hiccup of parameters - would be the falling cylinder-head or coolant temperature and yaw and decay of airspeed.
Having flown large supercharged piston engines professionally, and having worked on the same for a number of years as a mechanic and inspector, as well as having overhauled, installed, rebuilt, and maintained both the propellers and governor assemblies for those engines, I'll allow that you're substantially correct in general.
Several posters have touched on correct principles, and some have been fairly misguided by partial facts.
A propeller governor, especially for a Hamilton Standard Hydromatic and other types of propeller systems found on large radial piston engines, doesn't work on torque, but strictly on RPM. The amount of torque produced by the engine will determine the pitch commanded by the propeller governor, in it's attempt to maintain a particular RPM...but it won't determine the RPM.
Until the propeller reaches it's high or low pitch stops, the propeller and engine will maintain a constant RPM (assuming no governor or stephead malfunctions). Once the propeller comes to rest on the low pitch stops, it's in an underspeed condition and it's RPM is a function of airspeed (assuming no engine power, or in other words, assuming no torque). If the propeller is resting on the high pitch stops, then it's reached an overspeed condition and it's RPM is a function of both airspeed and torque...engine power.
Whether the RPM gauge will indicate an engine failure is largely a function of the engine RPM at the time of the engine failure. It may, or may not show a loss of RPM. At a low airspeed, a loss of engine power may result in an RPM decay, because if the propeller is already resting on the low pitch stops or reaches them in an effort to maintain RPM via the governor, nothing can be done to keep RPM up once airspeed is no longer sufficient to drive the propeller at it's scheduled RPM. Therefore, at lower airspeeds, one may see a loss of RPM following an engine failure or power loss. At higher airspeeds, quite likely no loss will be seen.
Oil pressure often remains constant following power loss, particularly if the only change has been cessation of fuel flow. If the engine continues to turn at the same RPM, then oil pressure remains constant.
Whereas the original poster said nothing about a BMEP (Brake Manifold Effective Pressure) gauge, it need not be addressed here. It will show a power loss, but as it hasn't been included in the instrumentation stipulated in the original post, it's irrelevant.
Manifold pressure is a more complex subject, and may or may not show a decrease. The subject powerplant in the original post was equipped with a supercharger, which is geared to the engine and has an output based on throttle position and engine RPM. Whether one sees a manifold pressure drop following a power loss depends on where the manifold pressure was to begin with. One may or may not see a drop. I've experienced engine failures involving disintegration of the supercharger clutch, for example, in a R2600, in which the manifold pressure merely reduced to ambient, or barometric. I've experienced other types of power loss or failures, especially at lower power settings, in which the manifold pressure indication remained the same.
Someone suggested that because the supercharger is geared to the engine, and because the supercharger is theoretically turning at the same speed as the engine, there will be no manifold pressure loss. This isn't true. Bear in mind that because of the use of a constant speed propeller, one can increase manifold pressure substantially above barometric, with the use of a supercharger, with no increase in RPM. RPM, therefore, is not the only determining factor regarding manifold pressure, when a supercharger is used.
I recall a P2V crash (using the R3850 motor) some years ago, involving not a power loss, but a governor failure. The governor couldn't regulate speed within limits. The crew could push the power up, but the propeller would overspeed, and power would have to be retarded. The airplane would descend. Power would be pushed up again, the airplane would accelerate, and the propeller would again overspeed. The power would be retarded, the airplane would sink...and this dance continued over the course of an hour or so until the airplane finally crashed on the high desert floor.
Had the crew realized what was happening, they could have flown slowly and still used engine power. The engine RPM was exceeding limits largely because it was being driven not only by engine torque, but by airspeed. Slow down, and while full power wouldn't be available, partial power would be...and the crew could have carried power on that engine, and used it to return to the airport and land.
EGT is an important indication, or other temperature indications to include CHT (not in all circumstances). Other indications such as an ignition analyzer may also prove useful, though on a large radial engine one usually is fully aware of the power loss by simple fact of assymetrical thrust, yaw, and the heavy rudder required to keep the airplane flying straight...as well as the change in sound, and fuel flow.
Fuel flow sometimes proves to be an excellent indication of engine health, especially in conjunction with EGT, but is particularly so in the case of the scenario presented by the original poster: fuel exhaustion.
Being able to look at the full picture and make a decision before attempting to mash a feather button is important, as it's easy to rush and feather the wrong engine...particularly on a piston-powered airplane.
In a single (no feather) you move the pitch control to full coarse in order to reduce drag and give you the best glide distance.
That really depends on the engine and propeller combination, and is not true for all aircraft, by any means.