Well here's something to consider reguarding Castrols new aviation oil. All APPROVED aviation oils must meet certain performance standards to get the FAA's blessing. In a nutshell that means they must all meet film/shear strength requirements, meet an antiquated anticorrosion standard and be compatible with eachother. Afterall, if your on a crosscountry trip and need 2 qts of Aeroshell 100 and all they have is Phillips 20W/50, the FAA says that they MUST be compatible so you can mix them. Most likely ALL aviation oils greatly exceed all of those requirements, afterall they are minimums.
In regards the the concern about use in our transmissions with the gears, etc. If Castrols new oil is approved for use in any radial engine or in geared engines like Lycoming GO-480's or Continental GTSIO-520's (which I suspect it is), I would say that it would be just fine in a Hiller. Being concerned about how long the oil sticks the the gears with loss of pressure (ie loss of engine)is really kind of a moot poind. They ALL will fling off in short order. Afterall, thats why under normal conditions you have them in a bath or spray and I don't think an oil whose spec was written 50+ years ago will stay on those parts any better than an oil which meets a more modern spec.
That all being said, to be technical, the ONLY approved oil for use in the Franklin powered Hillers is the oil specified in the lubrication order listed in the approved Hiller service manual. That being MIL-L-6082 SAE30 or SAE40 depending on temp. (ie Aeroshell 100). However I don't think you would get gigged by the FAA or have any problems if you used an approved oil that exceeded this rather old MILSPEC (ie Aeroshell 15/50, Phillips 20/50 or the new Castrol)
Just my 2 cents worth.