The designation of OM15/OX26 comes about though what they are ( O for oil) and what its base is (M stands for mineral oil, X meaning synthetic). Its all covered by the relevant Def-Stan regulations.
The strangest part about this whole "oh its was easier when the tins were different" is that lowly stackers manage to read the tins well enough to issue the correct stuff so surely the Cosford master race should be able to manage it.