Engine/Transmission (1994 - 1998) 15w40 vs 5w40

[dodge cummins]

ok what is recomended as far as oil. right now i am running mobil 1 5w40 diesel rated oil. talking to other people who are diesel minded say that it is actually better to run a thicker viscosity oil 15w40 when pulling heavy loads is this true? i was just reading old posts of people having trouble running amsoil synthetic oils because they were actually showing more wear on oil analysis. i want my engine to have as minimal wear as possible. i have noticed that since i switched to mobil 1 my soot control has been superior because i cannot detect any soot. upon cold morning startup i do not have any smoke anymore and i am thinking thats because of the oil. whats everyone else think? The real reason why i am starting to second guess the oil also is when i check the oil the 15w40 is always nice and thick and really coats the dipstick. when i check the oil with 5w40 you will actually see how much thinner it really is because it can sometimes drip off

 

[RustyJC]

The fact that you are using a SYNTHETIC 5W-40 oil puts a whole new spin on this viscosity debate.



With dino oils, a 5W-40 oil was (to oversimplify) made with a 5 weight base stock and then dosed with viscosity index (VI) improvers in order to achieve the viscosity of a 40 weight oil at 210 degF. Obviously, this meant that a 5W-40 dino oil had a much heavier dose of VI improvers than a 15W-40 dino oil. The problem with these VI improvers is that they were subject to molecular shearing, breakdown and degradation, so (again, to oversimplify), the less VI improver in the oil, the better.



Synthetics, especially Group IV polyalphaolefins (PAOs), have superior inherent base stock VI characteristics. Therefore, they don't need the heavy VI improver dosing like a dino oil to achieve the broad viscosity spread of a 5W-40 oil.



Rusty

 

[jmeehan]

Rusty,

That's some good info, thanks.

As far as the soot goes, if I can't see it in my oil I assume it's still in the engine gunking things up. I'd rather have it suspended in the oil, and drained out at regular intervals. Anyone have any thoughts on this?

 

[dodge cummins]

so basically speaking a mobil 1 synthetic 5 w40 is far superior than a dino 15w40 as far as over the road operation and towing?

 

[RustyJC]

Yep, although the Delo 400 15W-40 dino oil works just fine for me. I run synthetics in my other vehicles but choose to run the Delo 400 in my truck because of its unique operating profile.



Rusty

 

[dodge cummins]

well how is delo 400 dino oil so unique compared to say mobil delvac dino oil?(just trying to learn)

 

[RustyJC]

Chevron has an internal additives group (Oronite) with a lot of marine diesel experience burning some really trashy Bunker-type fuels. Their additive packages have proved especially proficient at corrosion control, ability to suspend and carry soot, TBN control, etc. A number of the other oils are refined or blended by their manufacturer who purchases additive packages from companies such as Lubrizol and others.



I have absolutely nothing against Rotella or Delvac 1300, but our truck is used almost exclusively to tow our 5th wheel RV. It will go out on a 500-1000 mile trip over a long weekend, then it may sit in the garage for 1-2 weeks. In addition to the soot, fuel dilution, combustion acids, etc. sitting in the crankcase (note that my truck has larger injectors), condensation in the crankcase can be a problem here on the Texas Gulf Coast just with ambient heating and cooling of the block under high humidity conditions. Because of this, I change oil and filter at 6-month intervals. Sometimes, the oil will only have 3000 miles on it when I change it, but my purpose in changing it is to flush out the contaminants in the oil - the oil itself really doesn't have enough miles on it to have deteriorated. Under this operating profile, the benefits of synthetic oil don't justify the costs of dumping it at such low mileage. Therefore, I run the dino oil that (IMHO) best fits my operating profile.



Rusty

 

[dodge cummins]

now my next question is i change the oil every 5000 miles. it doesn't matter to me if its synthetic or dino because no matter what it is,the oil still gets dirty. oil is cheap engine expensive. even though i run synthetic oil on regular maintence schedules compared to extended drain intervals doesn't synthetic provide some kind of advantage over the dino even changing it at regular intervals?

 

[RustyJC]

Yes, the synthetic undoubtedly offers operational benefits. Bear in mind, though, that there are lots of B-series Cummins that have gone 500,000 to 1,000,000 miles on dino oil, even in severe towing service.



My truck accumulates about 8,000 miles per year. At that rate, even if it only went 500,000 miles before overhaul, that would take 62. 5 years. Since I would be 116. 5 years old at that time, reaching 500,000 miles isn't really a concern for me.



Rusty

 

[Cooker]

Rusty did a good job at suming things up.



Personally I run a 5w40 only because I live in a colder climate and the synthetic 5w flows much better on a cold start than a 15w. The synthetic will handle temperature extremes much better and may yield slightly lower engine oil temp numbers under heavy towing situation.



If I lived in a warmer climate, I would definitely run Delo 15w40 as I feel it is one of the best non-synthetic 15w40 oils out there and could probably hold it's own very well against synthetics under a normal service interval.

 

[dodge cummins]

hey cooker do you run 5w40 all year long even during the summer?

 

[Cooker]

Yep, I run it year round.