I dont think there would be much problems using a fully synthetic oil though I would prefer to use semi-synthetic for engines above 5 years old. However, this is a matter of personal preference.
The oil will not drain from the filter - not that I have noticed any of this happening. I would be more concerned about the hoses and cables going brittle from storage. This was what I found when storing my brother's W123 ... that the hoses went dry and brittle. I needed to run the car more often than once a fortnight just so that the juices could be pumped up and lubricate every possible hose and cable there is!
