Synthetic oils designed for extreme aviation use are manufactured via chemical processes such as the Fischer-Tropsch process to ensure stability at extreme temperatures and extreme operating conditions.
They have none of the natural contaminants and impurities of lubricating products that are manufactured using crude oil bases.
As such, they are as chemically stable, if not more stable, than any other oil. I see no reason to become concerned about degradation after storage for several years.
If there was concern about oil degradation over modest time periods, you can guarantee there would be considerable warnings posted by aircraft and engine manufacturers, and aviation regulatory authorities, about potential problems... and these warnings would be easily found.
I can find no such warnings, so I think you can rest easily that your valuable RR 250-C47B won't suffer any deleterious effect from using an oil that has been in storage for some time.
If still in doubt, consult the engine manufacturer, I'm sure that they will have worthy advice on the subject.
The storage method is the critical factor. Properly-factory-sealed containers should have no concern attached to them at all. It's when containers have been opened, and then stored, is when concern should be raised.
The contamination of oil by moisture (purely from the air, via condensation) and by other physical contaminants such as dust and dirt, is what destroys oil. If the oil is in a factory-sealed container, the possibility of these contaminants reaching the oil is virtually nil.
The reason that some oil companies place a shelf life on oil, is because they have no control over storage methods, temperatures and conditions, once the oil is sold. Thus, they are obliged to place shelf life limits on oils to protect themselves from claims.