Been using schaeffers 5w40 full synthetic was wondering if there was much different between that and the 15w40

 

What oil is specified for your engine? The first number is how the oil will flow when cold so a “15” will be thicker than a “5” when cold, not great for start-up if your engine tolerances are designed for a 5w.

 

5W flows better when engine is cold. Both are 40W at operating temp. 5W-40 seems to be the better choice, and that is what I run 365/year.

 

15w-40 is typically very shear stable so if you are in the south and your ambient temps are above freezing, I think a 15w-40 would be fine. It would certainly save you a bit of money and provide great protection. If you're here in the north, the 5w-40 provides better flow at cold at start-up and is a little easier on your starter as well. I run 15w-40 year round in the F450 because I rarely use it in the winter and it leaks a little less than the 5w-40. I plow snow with my tractor so I run a 0w-40 or 5w-40.

 

It isn't a huge deal, but as mass production machining becomes more precise the requirement for less viscous oils increase to provide the required fluid barrier. Most the time, viscosity recommendations are based on EPA requirements as seen in the GM 6.2L engine recall requiring a higher viscosity engine oil.

 

This is something I don't think most people are up on - mass production has gotten better and more precise over the last few decades. OEMs are using extremely tight tolerances to build things, thus requiring less viscous oils but more of it. For instance, my parents' old 350-powered Suburban used 5 quarts of 10W30...my 3800 Series II used 4.5 of 5W30 despite being a smaller motor...my wife's Expedition with the 3.5 Ecoboost uses 6 quarts of 5W30. I've had others requiring 5W or 0W20.

My FIL is one of those guys who wants to put 10W30 or 15W40 in everything...and a lot of newer engines just don't need it.

 

Everything said above said being true

i did find not to my surprise that according to the motor oil geek , his test verified by oil samples found that oil with a higher cold number 15w, 10w vs 5w had lower metal particle wear indicated in the oil samples . His conclusion was that thinner oil rolls off metal surfaces quicker when the engines are not running ….meaning this was all indicative that thinner oil is more prone to dry starts.


and as was said earlier, these super thin oils give the OEM CAFE points for their CAFE fuel economy goals…thinner fluids provide more mpg ….who cares that wear beyond 5 years is accelerating.

 

15W-40 generally is a more viscous oil at all temperatures than 0W-40, 5W-40, and 10W-40 oils. That may lead to a thicker fluid oil film on parts at full operating temperatures and resist wear better.

There is a viscosity range for a particular grade of oil. For example, SAE 40 ranges between from 12.5 to less than 16.3 centistokes (cST) at 100 degrees centigrade. An oil at 12.5 cST and an oil at 16.2 cST at 100 C are both going to be rated as 40 weight but the first one is barely thicker than SAE 30 and the other one is just shy of 50 weight. Secondly, and this is specific to SAE 40 oil, 15W-40, 20W-40, 25W-40, and straight grade 40 must be at least 3.7 cST at 150 C (similar to SAE 50 and 60) while 0W-40, 5W-40, and 10W-40 must only be a minimum of 2.9 cST at 150 C, which is identical to SAE 30 oil. It is more difficult to make a high viscosity index oil- one that has a lower low-temperature rating for the same hot-temperature rating (e.g. 0W-40) than one with a lower viscosity index (e.g. 15W-40.) Thus the oils with a high viscosity index tend to have a cold viscosity at the higher end of grade and a hot viscosity at the lower end of the range- so that 0W-40 may only be slightly different in actual viscosity than a good 5W-30.

Usually when people talk about super-thin oils they talk about the hot-temperature grade being low, such as Ford and Honda using 5W-20 instead of 5W-30 a couple decades ago, and now it's Toyota using 0W-8. There is definitely a large component of trying to gain a tiny increase in fuel economy by running an oil with a very low hot viscosity. (Same thing's true with on-road heavy-duty diesels, oils with a 30 hot grade are becoming more popular than 40 grade for a few percent better fuel mileage.) Oils with a "normal" hot-temperature grade and a lower low-temperature grade like 0W-40 are mostly marketed as making it easier to start an engine in the cold compared to traditional oils like 15W-40.

Tolerances have tightened in many engines and tight tolerances usually require a lower viscosity oil. The way we are going, we might be seeing jugs of Mobil Velocite on the shelf instead of Mobil 1 before too long...