Correct you can go on the thicker side of the grade or the thinner side of the grade, not make your own. My point is people often think they made a 5W25 or 7.5W30, etc. etc. and they didn't. My suggestion is if someone wants a thicker 20 grade oil, or a thinner 30 grade oil, etc. buy one.You sort of can. You can't technically make a 5w25 but you can make a thicker 5w20 or a thinner 5w30.
20 grade oil @ 100C/cst minimum is 5.6 maximum is 9.3
30 grade [email protected] 100C/cst minimum is 9.3 maximum is 12.5
40 grade oil @ 100C/cst is minimum 12.5 maximum is 16.3
Where one grade ends another begins, all you go do is hope to get to the higher or lower end of the range by mixing. machinerylubrication.com has a nice chart, and shows the cold cranking limits as well.