EN48 - Aeroshell Oil 560 corresponds to Mil Spec MIL-PRF-23699 Class HTS. The Mil Spec shelf life recommendation for MIL-PRF-23699 Class HTS oils on the following site is 5 yrs.
Royal Lubricants - Synthetic and Petroleum Based Lubricating Oils, Hydraulic Fluids, Greases and Preservatives
This following site has a "Product Shelf Life Data" for Aeroshell synthetic turbine oils that leads only to the product MSDS... which does not list shelf life.
However, the MSDS does list the product as "stable" and to be stored in conditions of between -50 deg C and 50 deg C.
Aeroshell Turbine Oils
Possibly the most comprehensive information comes from this Shell Aviation lubricants site, which is quite detailed on storage methods and retesting. And yes, this site says 4 years shelf life, for Shell Turbine ester-type oils.
Note that the 4 years is the period set for re-testing of the oil, not necessarily the period before discarding it for aviation use.
http://www-static.shell.com/static/a..._shelflife.pdf
However, Shells summary reads (and I cut and pasted the wording)...
"In general, AeroShell products are inherently stable. If stored properly, their quality, properties and performance should not be affected by prolonged storage."