Rotella Oil
Senior Member
Joined: Jan 2012
Posts: 952
Likes: 4
Year: 2004
Model: Grand Cherokee
Engine: 2000 4.0
Herp Derp Jerp
Joined: Nov 2011
Posts: 18,251
Likes: 17
From: Parham, ON
Year: 1999
Model: Cherokee
Engine: 4.0L OBD-II
Seasoned Member
Joined: Dec 2019
Posts: 257
Likes: 11
From: Los Angeles
Year: 1992
Model: Cherokee (XJ)
The misconceptions! You need oil Jesus.
Synthetic oil is NOT thinner. For example: 5w30 dino and 5w30 synthetic are the same viscosity. 10w40 dino and 10w40 synthetic are the same viscosity.
Moving on to what multi-viscosity ratings actually mean...
5w40 means that when COLD (0 deg C) its viscosity is that of a COLD 5 weight. When HOT (100 deg C) its viscosity is that of a HOT 40 weight.
Thicker is not inherently better. Long, technical story short, where oil viscosity is XwYY, X should be as low as possible. Even a cold 5 weight is still much thicker than a hot 40, so some people's belief that 0w__ and 5w__ oils are too thin for the 4.0 is false. YY can be 30 or 40, but 40 has been proven to not be inherently any better for the 4.0 than 30.
Now, you think that gear oil is thicker than motor oil because gear oil has ratings like 80w90. Gear oil isn't actually that much thicker. The ratings are so much different though because they are rated in a completely different way for different temps/circumstances. Regardless, you should NEVER use gear oil in the engine.
By running 15w40, your oil is needlessly thick when cold which could be producing increase wear during warm-up. Once it's hot, it's a 40 weight, and that is OK, but ultimately unnecessary. A 30 will protect just as well or perhaps even better and could provide a slight MPG boost.
Quoting myself from a thread on a different forum:
Synthetic oil is NOT thinner. For example: 5w30 dino and 5w30 synthetic are the same viscosity. 10w40 dino and 10w40 synthetic are the same viscosity.
Moving on to what multi-viscosity ratings actually mean...
5w40 means that when COLD (0 deg C) its viscosity is that of a COLD 5 weight. When HOT (100 deg C) its viscosity is that of a HOT 40 weight.
Thicker is not inherently better. Long, technical story short, where oil viscosity is XwYY, X should be as low as possible. Even a cold 5 weight is still much thicker than a hot 40, so some people's belief that 0w__ and 5w__ oils are too thin for the 4.0 is false. YY can be 30 or 40, but 40 has been proven to not be inherently any better for the 4.0 than 30.
Now, you think that gear oil is thicker than motor oil because gear oil has ratings like 80w90. Gear oil isn't actually that much thicker. The ratings are so much different though because they are rated in a completely different way for different temps/circumstances. Regardless, you should NEVER use gear oil in the engine.
By running 15w40, your oil is needlessly thick when cold which could be producing increase wear during warm-up. Once it's hot, it's a 40 weight, and that is OK, but ultimately unnecessary. A 30 will protect just as well or perhaps even better and could provide a slight MPG boost.
Quoting myself from a thread on a different forum:
Also just to confirm, if I run rotella 0w40 over 5w30 in my cherokee, the 40 weight won't be too thick?
Thanks
Newbie
Joined: Apr 2021
Posts: 17
Likes: 1
From: Ohio
Model: Cherokee (XJ)
The misconceptions! You need oil Jesus.
Synthetic oil is NOT thinner. For example: 5w30 dino and 5w30 synthetic are the same viscosity. 10w40 dino and 10w40 synthetic are the same viscosity.
Moving on to what multi-viscosity ratings actually mean...
5w40 means that when COLD (0 deg C) its viscosity is that of a COLD 5 weight. When HOT (100 deg C) its viscosity is that of a HOT 40 weight.
Thicker is not inherently better. Long, technical story short, where oil viscosity is XwYY, X should be as low as possible. Even a cold 5 weight is still much thicker than a hot 40, so some people's belief that 0w__ and 5w__ oils are too thin for the 4.0 is false. YY can be 30 or 40, but 40 has been proven to not be inherently any better for the 4.0 than 30.
Now, you think that gear oil is thicker than motor oil because gear oil has ratings like 80w90. Gear oil isn't actually that much thicker. The ratings are so much different though because they are rated in a completely different way for different temps/circumstances. Regardless, you should NEVER use gear oil in the engine.
By running 15w40, your oil is needlessly thick when cold which could be producing increase wear during warm-up. Once it's hot, it's a 40 weight, and that is OK, but ultimately unnecessary. A 30 will protect just as well or perhaps even better and could provide a slight MPG boost.
Quoting myself from a thread on a different forum:
Synthetic oil is NOT thinner. For example: 5w30 dino and 5w30 synthetic are the same viscosity. 10w40 dino and 10w40 synthetic are the same viscosity.
Moving on to what multi-viscosity ratings actually mean...
5w40 means that when COLD (0 deg C) its viscosity is that of a COLD 5 weight. When HOT (100 deg C) its viscosity is that of a HOT 40 weight.
Thicker is not inherently better. Long, technical story short, where oil viscosity is XwYY, X should be as low as possible. Even a cold 5 weight is still much thicker than a hot 40, so some people's belief that 0w__ and 5w__ oils are too thin for the 4.0 is false. YY can be 30 or 40, but 40 has been proven to not be inherently any better for the 4.0 than 30.
Now, you think that gear oil is thicker than motor oil because gear oil has ratings like 80w90. Gear oil isn't actually that much thicker. The ratings are so much different though because they are rated in a completely different way for different temps/circumstances. Regardless, you should NEVER use gear oil in the engine.
By running 15w40, your oil is needlessly thick when cold which could be producing increase wear during warm-up. Once it's hot, it's a 40 weight, and that is OK, but ultimately unnecessary. A 30 will protect just as well or perhaps even better and could provide a slight MPG boost.
Quoting myself from a thread on a different forum:
Junior Member
Joined: Dec 2023
Posts: 75
Likes: 15
From: Los Angeles CA
Year: 1990, 1995
Model: Cherokee (XJ)
Engine: 4.0
I love replying to old threads, for historical interest.
I got a gallon of Shell Rotella T4 10W-30 Diesel Engine Oil on sale at AutoZone for $18 in August 2024 in Los Angeles, California.
It was on sale at Walmart for $16 but I would have had to drive farther.
I got a gallon of Shell Rotella T4 10W-30 Diesel Engine Oil on sale at AutoZone for $18 in August 2024 in Los Angeles, California.
It was on sale at Walmart for $16 but I would have had to drive farther.
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