Forgive me if I am repeating what somebody else wrote (or if I am repeating myself).
I find that when they come into my hands, most milk cars can use a basic servicing. I disassemble the mechanism as far as it will go by removing screws, e-rings, and/or horseshoe washers, and carefully clean all the parts, except the dashpot. (they are usually OK) If need be, I polish parts that slide over each other. I do not use any lubricant (Note that the Lionel service manual says not to use anything other than "Molykote powder" or graphite. )
If you do decide to disassemble to dashpot, be aware that putting too much grease on the dashpot piston will stop the mechanism from working properly. Either it will take a lot of voltage to get things moving, and when the mechanism finally does move, the cans go flying. Or the mechanism won't move at all.
