RMPC, do you know what the actual difference is between 15w-40 and 5w-40?

The numbers are the viscosity rating. The two numbers indicate that the oil is a multi-weight. Meaning that the first number is the viscosity at a cold temperature, and the second number is the viscosity at normal operating temperature.

At operating temperatures, both oils are EXACTLY IDENTICAL. The difference is the first number, which is the viscocity at cold temps. The 5w-40 will flow better at colder temps, which is why many folks no longer have cold start romps in the winter when using 5w-40 oils. Once both oils warm up, the viscosity is the same, meaning that neither oil will thin past the point of their rating.

So 5w-40 is only thinner and flows easier at colder temperatures than 15w-40. When both reach operating temperature, they both flow the same and neither one is thinner than the other.

 

Your conception of oil viscosity is upside down. All oils thin as they get hotter, the API viscosity rating for multigrade engine oils reports the oil's viscosity rating typically at freezing and boiling point of water. The wider difference in viscosity rating means in reality that the oil thins less as it gets hotter. So a 15w-40 oil actually thins faster than a 5w-40 oil. At temperatures above 212*, 5W-40 oil is thicker than 15w-40 oil.

Don't believe me? do a little research into viscosity index of oils. The higher an oil's viscosity index, the more slowly it thins with increasing temperature. Rotella T 15-40 has a viscosity index of 141, Rotella T 5w-40 has a viscosity index of 176 per Shell's web site.

 

Ok you guys win I havent looked into the Shell web site and maybe they've come up with some magic in the last few years... I was hesatant to use a multi grade ie: 15 40 when it came out...too, I just am living on 20 plus years of wrenching on diesels that drank straight 30 wt stuff and have tractors in the family that have many thousands of engine hours on em from the 60's and are still going and never had the pan off. I suppose I should go along with the 21st century but still feel in my gut that the old school stuff will last longer. Not everyone can afford a 50,000.00 truck with a computer making all the decisions. When it quits making the proper decisions and I have to walk home... I'm getting back into my mechanical engined truck thats been around for years, and will be around long after the electron brained one has bit the dust...I can fix my no start problems with a piece of bailing wire or string.... can you?

 

Sheeze!!

Okay, 5w-40 "T" synthetic is in there. So far, so good. We haven't had any really cold mornings yet to see how it affects start-up, but, in my various John Deere diesel stuff, there's noticeably easier cold-weather starts vs the 15-40 (various brands) that was in them before the change.

Thanks for all the responses.

 

??????
I always heard that to make a multi-viscosity oil you started with the thinner oils and added polymers that expanded when hot to simulate the thicker oils behavior. A 5-40 weight oil is just that: A 5 weight oil that has additives in it. Nothing wrong with that, but the difference comes in when the oil reaches the end of it's service life. The polymers get sheared down and you no longer have 5-40 but more like 5-20 weight oil.
It's made me very disciplined about frequent oil changes and not really caring whether I have a bypass filter or not. I change the oil way more often than Ford recommends to keep everything fresh.

 

yes its five times the weight of water at start up and forty times the weight of water at 100 degrees F.

 

Dan, I think that was a bigger problem when multi grade oils first came out. The technology now keeps those polymers from breaking down as easily to maintain the oil viscosity for a longer interval. Eventually they do break down, no matter what the weight of the oil, and thin out over time. Synthetics tend to resist breakdown for longer periods of time, simply because of the nature of dino oil itself. Running an oil analysis like Blackstone will tell you how your oil is holding up and how long you can safely run you oil before it breaks down too much.

 

That's true for multi-viscosity oils but not for synthetics. Which is why Rotella 5W-40 synthetic (what we were discussing) is thicker at higher temperatures than 15w-40. Synthetics are also very resistant to shear.

That's funny, I always thought oil was lighter than water, that's why it floats. Forty times the weight of water, that would be like 360 pounds per gallon. Heavy stuff.

 

Viscosity of Synthetic Rotella 5w40
40(Degrees C)=90
100(Degrees C)=15.0
Neutralization=11.5

Viscosity of regular (Dino) Rotella 15w40
40(Degrees C)=120
100(Degrees C)=15.5
Neutralization=10.1

Sounds like the Synthetic (Pronounced Sin-thet-ick=not extra refined dino as noted in a previous post) is superior in terms of reasonably lower startup viscosity and is nearly the same viscosity at operating temperatures, not to mention that it has a higher capacity to neutralize the resulting combustion acids - which would theoretically mean slightly longer oil change intervals with better protection. Synthetics are more resistent to prolonged high temperature running.

 

I seen one on ebay when I was looking for a Truck that was a 05' with 500,000 miles on it I did not believe it at first but he showed a pic of the odmeter and sure enough it had 477,695 on the odmeter. And that was put on in about a year and a half.

 

That's funny, I always thought oil was lighter than water, that's why it floats. Forty times the weight of water, that would be like 360 pounds per gallon. Heavy stuff. [/QUOTE]


come on "clux" viscosity not weight this discussion doesn't need to be as scientific as everyone is making it out to be.

 

hmmm....


That's over 873 miles a day. That's nearly 16 hours of nonstop running at 55 miles an hour, with no stops for traffic or refueling. I'm gonna have to whip out the ole' flag on that one. Photoshop or a tampered odometer.

I've tampered with an odometer when returning a rental car in Hawaii after a trip to Shanghai, and I used photoshop on my Oxford transcript when applying to grad school at South Austin Tehcnical Community College of Arts, so I know both can be done

 

yellermax you may be on to something. but remember the guy problay got the truck as early as may of 04, and thats two years and four to five monthes. so thats about 585 miles a day (still unbelievable).

 

True. Also, for two and a half years, if you could get to 300 miles an hour you would only have to drive for an hour and fifty minutes a day.

 

Yanking your chain, man. But I think viscosity is scientifically measured as a function of the amount of time it takes a fluid to flow through a certain size aperature at a certain temperature (I can't remember what temp), not compared to water. But you may have your onw way of doing it.