What about tagging the articles with GR date, then the user has at least optically a help to pick the right one, even you can't differentiate it in SAP.
If you really have the critical requirement to issue the oldest stock first then you should support this requirement with batch management.
Educating users usually helps a lot too, I am pretty certain they change the dinner plans at home if they see that they have something in the fridge which has to eaten today or has to go to the waste bin tomorrow.