"F" and "E" on my oil filler cap? Why?

Those indicate the vendor that supplied the part.

This is required for all OEM parts so that if there's a problem, it's a short Guilty Bastard Checklist.

Edit, no, I don't know the vendor, but can be internal code. For example, FE could be Federal Mogul, model "E", or it could just be FE means 'Stant' or 'TRW'.