Hi Michael,
This subject rears up occasionally re. OM13, I would be interested to know how much a 25 litre drum of Aeroshell No.1 costs, as anything related to aviation usually comes at a premium and of a standard in excess of the requirements needed for a fluid flywheel or Saracen brake and steering system. I am well aquainted with the army oil designations having been involved with military and their vehicles for many years. I once spoke to Morris Lubricants and they advised to use their Liquimatic oil of ISO10 grade, which I have used over the years in a range of vehicles with no problems.
Incidentally, OM15 is a "superclean" oil not issued to the army, only Navy and RAF as it is for aircraft hydraulics.
regards, Richard