Richard is correct, only Fox's U-bet syrup (4 squirts) is proper. Additional and essential requirements: A 10 or 12 ounce c u r v e d glass (GLASS), and no seltzer dispenser with a weak delivery. Anything else is just a watered down chocolate milk. Must be served with a straw.
Important additional and essential details for a proper Egg Cream: 12 ounce curved soda fountain glass (GLASS), and as Richard pointed out, Fox's U-bet syrup.