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You sort of can. You can't technically make a 5w25 but you can make a thicker 5w20 or a thinner 5w30.
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.

EG:
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.
 
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They keep making it thinner to improve CAFE by 0.1 mpg

Started as 5w30, then 5w20 and now 0w20
Spot on!
Image below is from the 2012 Jeep User Manual then on 2013 and on same engine same everything FCA switched their recommendation to 5W-20 for the reason you stated.

 

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Amsoil for all of my Jeeps.
XJ 10w30
JK 5w20 winter, 5w30 summer
JL 0w20
I live in WI and run AMSOIL Boosted 5W-20 year-round and AMSOIL in every other hole I service. 60K and other than a slight tick no issues inside the engine.
 

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I only run straight marvels mystery oil, my engine makes mysterious sounds now so I can only assume that means it's working as advertised.
 
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I only run straight marvels mystery oil, my engine makes mysterious sounds now so I can only assume that means it's working as advertised.
Excellent choice! 4.9 viscosity, with great cleaning ability. It should make the internal engine parts sparkle..lol
 
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