I just went out to wally world and bought 2 5qt containers of Mobil 1 Truck & SUV 5W40 oil. The back of the container says that it is good for all weather temps (I forget the exact range) for diesel engines.

This is from my Duramax supplement owners manual:

• SAE 15W-40
As shown in the viscosity chart, SAE 15W-40 is
best for your vehicle. However, you can use
SAE 10W-30 at temperatures above 0°F (−18°C).
When it’s very cold, below 0°F (−18°C), you should
use SAE 5W-40 to improve cold starting.
These numbers on the oil container show its
viscosity, or thickness. Do not use other viscosity
oils such as SAE 10W-40 or SAE 20W-50. According to this I should only be using 5W40 if I'm below 0. Well I'm in NJ and the summer is on its way - 90 degree days are coming. Should I return it and get 15W40 oil? What would happen if I ran the 5W40 weight oil?

Thanks..
Denis

You will be more than covered with the 5w-40 all year round. What the manual states is that you should run the 5w-40 in the winter to help aid with starting and cold oil flow. The M1 T&S 5w-40 is a full syn. where as the 15W-40 that is rec. is a dino oil. Just make sure that you allow enough miles to ensure engine break in before switching to syn. oil.

Hi Dennis,

Check the graph on page 5-19 of your '03 Duramax Diesel Supplement manual.
The chart shows 5W-40 as being OK for the higher temps and being an advantage in the colder weather.
I'm using 5W-40 Rotella Symthetic from Wally World and plan to use it year round. Mostly hiway driving and when I do pull its only 7,000lbs or so. And yes believe it or not here in S Ontario we'll have those 90 deg days too. I'll also travel south into those hotter days...but the graph says we're OK.
The 5W-40 was great on those few -30deg mornings back in Jan!
Richard

how many miles does your 2003 duramax have on it?

55k kms or 34k mi on mine.

Richard

We discussed this a while back. IIRC someone figured out that Mobil 1 Truck/SUV was rebadged Mobil Delvac 1 oil, which is an excellent oil. You chose wisely....:cool:

On EDIT: yep...went back and searched, and they are the same oil. Packaged different to appeal to the SUV crowd.

Tom

I have 23K miles.
I just ordered a couple of baldwin oil filters and plan on changin my oil this weekend.

So it looks like 5W40 will be safe..

Thanks everyone - my first diesel.. don't want to make a mistake!

I've been running syn in my truck(03) since around 8,000 miles, currently at 44,000. I used to switch back and forth summer and winter, but since the M1 comes in 5 qts now and the price is a little lower it's just too easy to use(no measuring or make quarts need to be added/purchased). Haven't had an engine failure so I'm think I'm doing it good.

How many miles should I rack up before switching to synthetic?

You will be more than covered with the 5w-40 all year round. What the manual states is that you should run the 5w-40 in the winter to help aid with starting and cold oil flow. The M1 T&S 5w-40 is a full syn. where as the 15W-40 that is rec. is a dino oil. Just make sure that you allow enough miles to ensure engine break in before switching to syn. oil.

Allow anywhere from 8,000 up towards 15,000 miles. This will depend on how hard you run the engine. Pulling big heavy loads will help seat the engine in quicker than just cruising around town. The only downfall is if the syn is put in prematurely it just will prolong the breakin period is the only side effect.