What is the difference between 15W40 and 5W40?

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I have no idea what the numbers mean. I do know that the higher the last numbers the thicker. Which oil is thicker? 5W40 or 15W40 and does it really make a difference if which one is put in a Ford 6.0 Powerstroke Diesel? They recommend 15W40.
 
In California is does not make much of a difference. In Michigan you would have to use a block heater if you wanted to run 15W40 this time of year while a 5W40 would crank and pump just fine with no heater. Imagine it like this the first number is how the oil will behave in the cold winter temp.'s. So a 5W will behave like a 5 weight oil dureing cold cranking. A 15W oil will behave like a 15 weight oil.

The other thing to bear in mind is that most 5W40's will be either fully synthetic or partialy synthetic while 15W40's will be plain dino oil. The additive packages though among the same brand will be either simalar or identical.
 
15W-40 will usually be Conventional Dino or Synthetic Blend.
5W-40 will usually be a Full Synthetic or Synthetic Blend.

The W number is a cold cranking and pumping standard that oils meet. 15W is usually pretty good down to 15*F. 5W is usually okay down to -15*F. Any lower and you need a 0W

Normally the Dino Conventional Oils are fine at factory service intervals. You can go longer with synthetics. If you dont deal with low temperatures a good 15W-40 Conventional like Chevron Delo 400, Mobil Delvac 1300, Motocraft Diesel.

Just make sure you always buy a HDEO (Diesel) as there are a few gasoline only 15W-40 out there and a fair number of 5W-40 Synthetic Gasoline Only Oils.

The 6.0 Powerstoke uses oil to run the injectors.. They can be picky about the oils they like. You may want to consult some of the internet forums for Powerstroke Enthusiast to see what the latest word is on preferred oils.
 
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