thought that DVD-18 are Warner speciality - Superman the Movie is on DVD-18 and packed in snapper case

I don't think Warners has really used DVD-18s since they started doing 2-Disc Special Editions. Universal is a studio that seems to pride itself on squeezing as much content on a single disc as possible. Up until a year or two ago, multi disc sets were fairly rare from them. They've been using DVD-18s recently for their television releases and on such titles as Schindler's List and the Casino SE. Double sided discs themselves aren't uncommon. DVD-14s have been commonly used by Fox and MGM.