oil-manufacturers don't do the same. Mobil does not. Royal Purple does not. Castrol does but does not list the 300Zx. Red Line does but does NOT list 0w30.
Amsoil's application guide is a lot more convenient, and it's easier than scanning a manual page and uploading it. Maybe some people don't have their owner's manual. What oil do you use? Why do you hate Amsoil?