Originally Posted by drhoward1988

Walmart brand deisel oil, dello by chevron, mobil 1, shell Rotella series, all good oils.

Originally Posted by mschi772

You need Oil Jesus.

Originally Posted by dcollins3208

I use T6 5w40 in almost everything I own. Wally world has it on the cheap. T5 10w30 will be just fine for your rig

Originally Posted by mschi772

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:

Originally Posted by mschi772

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: