That said, bearings are one of the few parts of the engine that really have a "too thick" penalty in terms of wear or life. The rest of the engine frankly doesn't mind if you run up a 50 or 60 grade at temps as low as 220-240. The thing is, the extra thick viscosity isn't doing anything helpful for you though. It's just robbing power and reducing the flow that cools many important parts of the engine. Once you have enough viscosity, adding more of it isn't lowering wear. You're just moving along the Stribeck in the same state of lubrication with less end less efficiency.
I think it's important to realize a unique aspect of the fan drive (which is shared with other prop drives): the peak load on the engine is always at peak RPM. Yes, on the dyno, the engine might show a torque peak well below rated power, but that's under a dyno load which is controlled independently of speed. With the fan/prop, the speed IS the load. If the fan was the dyno, you'd see peak HP and peak torque at the same RPM all the time.
So while the engine might be capable of 800hp or such, the actual power output is determined by the fan load (prop curve, drive ratio, etc). It's probably quite a bit less than the full dyno power the engine is capable of.
This means the engine will never see a high load at lower RPM. In my mind, that makes this application the textbook case of choosing viscosity by oil pressure at sustained high temp. If your oil is thick enough to keep the pump in regulation at peak temps in the application, then you have enough viscosity. Having more of it is doing nothing but reducing power.
I think a stout 30 grade is almost certainly sufficient here in terms of viscosity. IMO, the more important aspect of picking an oil here is the additive pack and base. I think a race oil (like HPLs BARO 5w30 or 5w40) would be warranted. I think an "optimal" peak oil temp would be in the 250F-270F range. With alcohol fuels, running too cold is a bigger problem than too hot. R4W is absolutely right, hotter oil is your friend here, and pick an oil that can handle the temp.
If the OP doesn't want to run a racing oil and wants perhaps a bit longer oxidation life, the other HPL oils I'd recommend would be
1) No VII 10w30 Euro
2) PCMO SAE30 or SAE40. (Straight grade is a real option for this case)
3) Supercar/Euro 5w40
The low key great option oil here is also VR1 10w30 synthetic. It's easy to find and it's proven. It just doesn't last long so you'll need to stay on top of oil changes.
I think it's important to realize a unique aspect of the fan drive (which is shared with other prop drives): the peak load on the engine is always at peak RPM. Yes, on the dyno, the engine might show a torque peak well below rated power, but that's under a dyno load which is controlled independently of speed. With the fan/prop, the speed IS the load. If the fan was the dyno, you'd see peak HP and peak torque at the same RPM all the time.
So while the engine might be capable of 800hp or such, the actual power output is determined by the fan load (prop curve, drive ratio, etc). It's probably quite a bit less than the full dyno power the engine is capable of.
This means the engine will never see a high load at lower RPM. In my mind, that makes this application the textbook case of choosing viscosity by oil pressure at sustained high temp. If your oil is thick enough to keep the pump in regulation at peak temps in the application, then you have enough viscosity. Having more of it is doing nothing but reducing power.
I think a stout 30 grade is almost certainly sufficient here in terms of viscosity. IMO, the more important aspect of picking an oil here is the additive pack and base. I think a race oil (like HPLs BARO 5w30 or 5w40) would be warranted. I think an "optimal" peak oil temp would be in the 250F-270F range. With alcohol fuels, running too cold is a bigger problem than too hot. R4W is absolutely right, hotter oil is your friend here, and pick an oil that can handle the temp.
If the OP doesn't want to run a racing oil and wants perhaps a bit longer oxidation life, the other HPL oils I'd recommend would be
1) No VII 10w30 Euro
2) PCMO SAE30 or SAE40. (Straight grade is a real option for this case)
3) Supercar/Euro 5w40
The low key great option oil here is also VR1 10w30 synthetic. It's easy to find and it's proven. It just doesn't last long so you'll need to stay on top of oil changes.