Difference between 0w40 and 15w40

[leanburn]

 I was having a bs section with a buddy and we started talking oil weights. From my understanding the first number represents the oils viscosity at colder temps and the seconds is at running temp. Is there any reason an engine can't run 0 weight oil year long? I know it's not necessary and more expensive but other than that ?

[YellowThumper]

When engine is cold you want the thinner oil so it flows throughout the engine more efficiently. As things warm up you need the heavier viscosity to provide enough of a barrier under normal load usage. Going with a single heavier viscosity would always be better than going the thinner. The thin weight is only for initial startup when everything internal is a bit tighter prior to heat expansion.

[leanburn]

So does that mean that you could run 0w40 in summer months with no ill effects ?

[rt green]

 the difference is if you pour 15w40 in a glass it would pour like 15 weight. 0 would pour like 0 weight. both will lubricate like 40 weight.

[leanburn]

Ok, so 0 weight oil year around no problem but then that brings me to another question.  If you require a 40 weight oil why would anyone bother with 20w40 vs 0w40?

[blown motor]

But don't the two weights become a homogenous mixture? It's not like they separate and pump separately. 

[msbaugh]

But don't the two weights become a homogenous mixture? It's not like they separate and pump separately. 

Lol.... Dude, it's not just two different oils mixed together.   Physical properties change as temp changes.  All oil thins with heat and thickens when it's cold. With multi weight it just changes differently and is tailored better due to the additives for start-up and different ambient temperatures.  It's still thicker when it's cold than it is warm, it's just that a 0w40 flows easier at 40 degrees F than a 15w40 at the same temp.  There won't be any issue running 0w40 in summer other than that you'll be wasting money.  I run 15w40 in my diesel most of the year and only change to the 5w40 if it's due during winter time

[dodj]

I've been running 5w40 in my diesel for 7 years now. Summer and winter.

[RCCDrew]

I'm no chemist, but is has to do with the different types and amounts of polymers in the oil. Polymers like to bind to things and keep them in suspension, and are the determining factor between single weight and multi weight oils, IIRC.

[blown motor]

Lol.... Dude, it's not just two different oils mixed together.   Physical properties change as temp changes.  All oil thins with heat and thickens when it's cold. With multi weight it just changes differently and is tailored better due to the additives for start-up and different ambient temperatures.  It's still thicker when it's cold than it is warm, it's just that a 0w40 flows easier at 40 degrees F than a 15w40 at the same temp.  There won't be any issue running 0w40 in summer other than that you'll be wasting money.  I run 15w40 in my diesel most of the year and only change to the 5w40 if it's due during winter time

And that's why I'm not a scientist. 

[quagmire]

Sheer stability drops dramatically as the spread between the two numbers increases.  This is the major reason why you cannot run 0w40's in most engines year round, or at least in areas that'll see extremely high ambient temps.

Basically, the oil viscosity index improvers responsible for making multiviscosity oils possible will "shear" and breakdown over time (how quick depends on drive cycle, ambient temps, engine design, load, etc.) and cause the oil to no longer be a 40 weight when hot.  So what started out as a 0w40 may quickly become a 0w30 or even a lower.  As that occurs, engine protection drops and reach a point where sludging, accelerated wear, and damage begin to take place.

This is a major reason why 10w40 was abandoned as a common weight many years ago.  Being conventional oils, they caused a lot of problems because the additives weren't stable long term.

Modern synthetics and extended life additives allow some of these extreme weight spread oils to exist, but they still are susceptible to shearing, just less so.

[msbaugh]

Then how are 15w40 and their 5w40 synthetic counterparts from delo, amsoil, and rotella the industry norm and best oils for diesel trucks and heavy duty applications? Ppl run oil analyses on these up to 10k miles without significant degradation or wear.

[quagmire]

0w40 has a large viscosity spread, 15w40 does not.  That's why 15w40 has always been a proven oil in HD applications.  You're subtracting the difference between the two numbers.  The larger the difference, the more viscosity improvers are used for a given oil type that can/will shear.

Also, I specifically stated that synthetics are less prone to this.  I'm not aware of any conventional 5w40 oils, ALL of the normal players in the HD diesel world are a synthetic blend at a minimum or more often a full synthetic.

Many HD diesel engines in trucks can go far more than 10K miles nowadays; 30-50K miles is possible in most modern engine designs with the correct oil filter and high MPG drive cycle.

[anlauto]

Man....I wanna say "15" so bad......

[msbaugh]

Makes sense. What do you mean say 15?