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Old 10-18-2012, 05:01 PM   #1
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5w-20 or 5w-30??

Which one do you guys think is best for 2008 JKU 3.8L?

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Old 10-18-2012, 05:04 PM   #2
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What ever the owners manual says is best. I think for the 3.8 it's 5w-30 but I can't say for sure

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Old 10-18-2012, 05:34 PM   #3
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If you're having any issues with oil consumption swap out your PCV and switch to 5W30. If not, the recommended weight is 5W20.
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Old 10-18-2012, 05:35 PM   #4
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5w-20.
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Old 10-18-2012, 07:01 PM   #5
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Your 3.8 fill cap says 5w20. The 2012's say 5w30, the 2013's say 5w20.
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Old 10-18-2012, 07:08 PM   #6
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Dealer has apparently been using 5W20 in my '12. Just realized that today. When spring comes I'm going to start doing it myself from now on with 5W30. The 20 weight stuff is like water at full temp.
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Old 10-18-2012, 09:20 PM   #7
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5W-30. Been doing it for years now. 3.8L tends to consume 5W-20.

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Old 10-19-2012, 02:59 AM   #8
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Quote:
Originally Posted by WXman View Post
Dealer has apparently been using 5W20 in my '12. Just realized that today. When spring comes I'm going to start doing it myself from now on with 5W30. The 20 weight stuff is like water at full temp.
My dealer put 5W20 in my 12's first oil change, I argued with them but they refused to change it back to 5W30 so I had to do that myself.
Seems they're not the only dealer not following the procedure and most people might not notice.
Considering the heat in So Cal, I will at least keep using 5W30 for summer.
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Old 10-19-2012, 05:27 AM   #9
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The cap on the Oil fill states 5w-20 on my 2009. I am not mechanically inclined so explain it to me like I am 5, but is there a real difference between the two?
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Old 10-19-2012, 07:17 AM   #10
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Originally Posted by southpaw75 View Post
The cap on the Oil fill states 5w-20 on my 2009. I am not mechanically inclined so explain it to me like I am 5, but is there a real difference between the two?
The first number indicates the weight when cold with a lower number being thinner which would flow easier. The second number is the weight when at operating temp.

5-20 would be the same weight and thickness as 5-30 when cold. At operating temps it'd be thinner thatn 5-30 thus flow easier and used for tighter clearances.
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Old 10-19-2012, 07:48 AM   #11
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Viscosity is not actually measured in "weights", but rather in units called "Stokes." If you're a famous scientist they name a unit after you. Stokes was a guy who worked on fluid flow. For oils, we use a hundredth of a Stoke, called a centi-Stoke, abbreviated cSt. "Weights" are a classification invented by the American Petroleum Institute (API). A different unit of viscosity, the centi-Poise, is used at very low and very high temperatures. 10 weight oil refers to oils within a range of viscosities, so two different brands of 10 weight oil might actually be quite different. 75 weight gear oil is actually about the same viscosity as 10 weight motor oil. Don't ask me why, I'm not a petroleum engineer.

AE W viscosity grades for engine oils
Grade cranking pumping
0w 3250cP at -30c 60,000cP at -40c
5w 3500cP at -25c 60,000cP at -35c
10w 3500cP at -20c 60,000cP at -30c
15w 3500cP at -15c 60,000cP at -25c
20w 4500cP at -10c 60,000cP at -20c
25w 6000cP at -5c 60,000cP at -15c

SAE viscosity grades for engine oils
Grade low shear high shear
20 5.6 - 9.3 cSt at 100c 2.6 cP at 150c
30 9.3 - 12.5 cSt at 100c 2.9 cP at 150c
40a 12.5 - 16.3 cSt at 100c 2.9 cP at 150c
40b 12.5 - 16.3 cSt at 100c 3.7 cP at 150c
50 16.3 - 21.9 cSt at 100c 3.7 cP at 150c
60 21.9 - 26.1 cSt at 100c 3.7 cP at 150c
a (0w-40, 5w-40, 10w-40 grades)
b (15w-40, 20w-40, 25w-40, 40 grades)
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Old 10-19-2012, 10:56 AM   #12
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Thanks, I learn something new every day.
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Old 10-19-2012, 09:05 PM   #13
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I took my 2009 Rubicon and my 2012 Sport to the dealer for oil change today. Service mgr is a friend and he told me that they stock both oils and put the specified oil in each model year. He told me that for warranty issues you want to use what Chrysler/Jeep recommends.
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Old 10-19-2012, 11:02 PM   #14
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I switched to synthetic mobil 5w-30 as it easy to find at costco. I hope I don't burn in the depths of dealership hell for this some day. 2010 JK, less than 10k miles.
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Old 10-20-2012, 07:12 PM   #15
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Most manufacturers are recommending 5w20 in newer vehicles these days for fuel efficiency reasons. Because of CAFE standards, the extra 0.25 mpg from the thinner oil makes a difference from their perspective. But keep in mind also that from their perspective your engine only needs to last until the warranty expires - so what's what's best for them isn't necessarily what's best for you or your engine. I've always used 5w30 for the extra protection it offers.

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