Ok, guys, I have a question about oil viscosity. I have an '04, LLY, it is my first diesel. I know, I know, but, I got to diesel as soon as I got tired
of fix'n the last gasser I owned, but then, I drove it for twenty years. I used to be a pretty good "shade-tree-mechanic", even an armature racetrack
mechanic, but, with the "modern" cars and trucks being designed by the "Gameboy" generation and pushing fifty, I was much less enamored with
get'n as greasy as the underside of an old jalopy and not being able to stand up straight for a week. I vowed to let the dealer do the dirty work when
I bought my new Chevy 3500, dually diesel. I have finally gotten tired of the dealer yanking me around and, I guess that I'm sort of missing dirty
fingernails. Besides, I never did trust another mechanic to work on my engines. Alright, long winded build up done, here is the question. I decided
to change my own oil this time and I know that the recommended is 15w / 40, but, I have run 20w / 50 in gassers for as long as I can remember. So,
I buys me a case of 20w / 50 Penzoil and toters home. Well, I thought, it won't hurt to at least peek at the owner's manual, just incase that there
was anything any I haven't seen before. Lo and Behold, It says, right there, DO NOT USE 10W / 40 OR 20W / 50. ??? What? It's Ok to use
15W / 40, 10W / 30, 5W / 40, (according to circumstances). It seems to me that 10W /40 falls completely within the viscosity requirements and
that 20W / 50 simply adds a bit of protection in hot climates, (I live in central Texas and the last time that it dropped below 80 degrees, I put my snow
shoes by the front door, just in case). What's the deal? I've had extremely good results with 20W / 50 in gassers. The last P.O.S. that I had was
an 84 Chevy V6. I ran 20W / 50 in it from the day I towed it off the new car lot! I personally rebuilt that engine at 256,000. Not because it was burning
any oil, but, because the valley gasket, (actually, the lack of vg), gave way and dumped the coolant into the oil pan. I had less than a .010 ring groove
on any cylinder. I drove that for another 125, 000. Does anyone understand the reason for not using 20W / 50? :badidea:

Well, first of all.:welcome:

I think the overwhelming reason they tell you not to use 20W50 motor oil in the duramax is this: Rating....If you look at the back of the one of the bottles of your 20W50, you'll see an API badge (usually a black and white circle) Now, I'm betting it has a rating of SL...but says nothing like CH...S_ ratings are for gas powered vehicles....C_ ratings are for diesels....someone with a lot more oil knowledge than I have can tell you what different additives are in the diesel (C_) rated oil that aren't in the gas (S_) rated oil, but I think that is probably the main reason..

Well, that's interesting. Sounds like I may have some studying to do. I wonder if there is a heavy weight, multi vis oil that would be suitable for diesel engines. Well, I knew that this would be a learning experience. Well, I'm off to exchange a case of oil.
Thanks for the info. I really hadn't considered that the additives might be different.

Well, that's interesting. Sounds like I may have some studying to do. I wonder if there is a heavy weight, multi vis oil that would be suitable for diesel engines. Well, I knew that this would be a learning experience. Well, I'm off to exchange a case of oil.
Thanks for the info. I really hadn't considered that the additives might be different.

As far as I know, 15w40 is the heaviest diesel rated oil out there.....

The Basics, CH rated oil has a high detergent content which is needed by diesel engines due to the high output of soot, (even with low sulfur fuels being required these days, ever look at your exhaust pipe???) as to the viscosity rating, I do not really understand the reason there is not a CH rated 20w/50 oil out there for diesels, but there is none that I know of. Must be for a reason. Maybe someone out there has the answer to this question.

Myself personally I use Chevron Delo 400 15w/40 oil in my truck and change my oil every 5000 miles weather it needs it or not. This of course are MY prefrences on oil. Make sure you get a good oil filter as well, pay a little extra for it, it'll payoff in the long run. I use AC Delco # PF 2232. Again, personal prefrence.

John

One issue often overlooked is the low/high weight ratio.
A 5w-30 has a 6:1 ratio, compared to a 3:1 ratio of a 10w-30
This means that the 5w-30 must have more viscosity improvers to hold 30w when hot. These VIs are one of the main components in oil breakdown. They are extremely sensitive to shear stresses, which are particularly high in wet-clutch motorcycle applications, DOHC engines, and turbochargers.

It is less of a concern with synth, and Rotella 5w-40, with it's 8:1 ratio, is probably still as good or better at holding it's high VI than a dino 15w-40 blend like Delo with it's 2.7:1 ratio, since synthetics do not require as many vicsocity improvers in the blend.

20w-50 is a 2.5:1, which is actually slightly better than 15w-40, but it is also a very heavy oil. I would be extremely hesitant to run anything heavier than a 15w in a turbocharged engine (I'm honestly surprised the recommend that heavy, I assume because of heat). I mean, the difference is noticeable. When I drained the factory Yamalube 20w-40 fill from my bike and switched to Rotella 5w-40, the Yamalube was like diff oil draining out, while the Rotella was like water.

If you have ever seen a diesel engine that has run on regular car oil the answer to your question would be answered very quickly. My grandfather like yourself thought why not but he never stopped and asked and he ran it in our old ford tractor. Next oil change there was chunks of black carbon coming out, 3 oil changes later(2hours on each) it finally cleaned up enough that we could put rotella back in it. Gas engines just don't need the detergents that diesels do.

Just did my first oil change today at 1800 miles.

Wow... that oil looked as bad as Mobil-1 did coming out of my Tundra at 8k. I was surprised at how black it was. DIC indicated 80% remaining. I installed Rotella 15w-40. I'll probably switch to synthetic at the next change.

Front diff, oil came out looking like it had water in it... very much like chocolate milk, or coffee with cream in it... even had white streaks like the thick part of the cream. Rear diff looked good.
Both magnets had a fair amount of crap on them, but I expected that.

My front diff fluid looked the same as yours, like it had water in it. Friend of mine said that the synthetic that GM is using does that.

also, should use lowest viscosity to maintain oil psi for engine op range. if you use higher engine works harder to pump, reducing HP. have also seen engines sheer oil pump drive from 50wt and loads up oiled surfaces(pistons, rings, bearings, crankshaft). have ran gensets on 30 wt continuous load, never had problem. the detergent and additive pkg for diesel engine is what gives long life. like theferm said.

the S in the gas based oils is for "Spark ignition engine" the C is for "Compression ignition engines"