Moving to 10W-30 or 5W-30 oil

Run 15-40 during the summer, 5-40 for the winter, here...

 

I've ran 5w30 amsoil series 3000 syn in a 1999 c12 year round with no problems.
I picked up 1/2 a mpg after changing oil pulling the same load.

 

Here it is: you would almost never, I did say almost and never correct?, want to lower the the 2nd set of numbers: 15w40. You can lower the first set but not the 2nd set. Here is why: if you run your truck hard, and are trying to make as much money as possible and not turn down heavy loads than you will need the oil that will withstand the higher temperatures and that what the 2nd set of numbers "mean" - max temp. Now, you can try to lower the first set of numbers but, you need to make sure that your engine is not going to burn it. Meaning oil passing by your piston rings and getting into your combustion chamber - talking from the current experience.

 

Almost forgot : if you running the cummins isx: you'll notice that your engine has the hose coming from the top front of your valve cover and is going to the bottom of your engine, not getting into real tech terms here. If your oil is thinner than it needs to be, it will end up, some of it, drain from the hose onto the ground. Next, you need to make sure that the oil pump can pump the thinner oil and not "spin tires in the water". I'm currently, summer time, running 5w40 and I've ended up adding 3-4 gallons of oil within first 10k miles. Changing it to 15w40 as soon as I get home.

 

now this just makes sense! so you sir, have no place on our forum. j/k of course.

 

I'm just trying to be helpful . I can always say use 0w40. Lol

 

Got to take issue with one of your points. The second set of numbers is not "max temp". Check with API... the first number is the winter flow rating and the second is the weight of the oil. A 5w40 and a 15w40 are both 40 weight oils. Just that the molecular structure of the 5w40 gives it a better winter flow rating.

Regarding the 10w30 or 5w30 discussion, the full synthetic diesel oils in this grade will do just fine, but that depends on the engine. I would only go that route on a younger engine. Else, as was recommended by others, go with a 5w40 or just stay with a 15w40. Older engines can tend to "shear" the molecules in the oil, and break down the viscosity. A newer, tight engine can use a 30 weight oil and not have any issues. Regardless, a xxw30 weight oil will hold up in a good engine even in hot temps under load. There has been more than enough testing done out there to show this. Oil technology has come a long, long ways in the last few years. If memory serves, I believe an outfit by the name of Jeff Foster Trucking, out of Superior, WI or thereabouts has been using a full synthetic 5w30 in a lot of their trucks for a while with decent results. They pull vans and flats.

Since doing this virtually means going to a full synthetic, you have to then determine if the results you are getting are worth the added cost. Just like different rifles of the same caliber do not all shoot well with the same brand of ammo, engines can sometimes have a preference in brand or grade of oil they get. I tried full synthetics at one point in my ISX, and never got good enough results to justify the added cost. Now, my diesel Jeep and my New Holland ag tractor love full synthetics. Why the ISX didn't realize a real benefit, is anyone's guess. It does like a synthetic blend I am using now.

Only way to really know for sure, is to try out the oil you are considering and run oil samples on it to see if it is working out. If you engine wear numbers go up, oil consumption increases, and the oil is breaking down, then you probably want to rethink using the stuff.

 

In my experience synthetics, did not work well in N14`s, an engine is made to run with a certain weight of oil to be able to withstand the stress loads of the different pieces, rod bearings especially.
I have a operating and maintenance manual, that covers C10, C12, 3406E, C15 and C16 it says it is OK to use 0W20 if the maximum temperature outside is 50F.
0W30 to 86F
0W40 to 104F
5W30 to 86F
5W40 to 104F
10W30 to 104F
and 15W40 to 122F
I looked in my Detroit manual and it said nothing that I could find, and I was amazed to find it in the Cat manual.
I would be very leery about using a thinner weight oil, The parasitic loss from using 15W40 rather than a lighter grade to me is just not worth the risk.
If you do it please keep us informed.

 

I had my motor overhauled the beginning of this year and cat filled it with 10w30. I asked them about it and they said cat recommends it in the acerts. I made a little better than a million miles the first go round on 15w40 so I went back to it. Not to sure about the lighter oil in the summer.

 

CAT recommends alot of things. I would have to think if you got a million plus miles on 15w40 oil I would go back to it. In your case its done its job.