Diesel engine oil

I have a 2004 DP with a 400hp Cat C9 and faithfully change oil each season. Manual say use 30W CJ type engine oil, so I have been using Delo 400 30W. This year I am finding auto parts store are stocking 15W- 40W and not the 30W. We live in Washington state and travel to Arizona for winter. So is it OK to switch to multi weight oil?

15/40 is just fine for diesels.  Personally I go with full synthetic.  Cost a bit more, but longer oil changes make up for it.

15/40 should have been used all along, easier starting and less wear on parts, better mileage.>>>Dan

My MH has the C9 Cat in it also. I use 15/40.

I agree with using 15/40

I am a fan of Rotella Oil, and you can get at Walmart or Menards

Read the label of whatever you get, make sure it meets CJ specifications.

Since this is a Cat engine, I'll disagree a bit.

15W-40 is a standard choice for Cummins engines in regions where the temps dip below freezing occasionally.  Plain SAE 30 is generally good down to about 32 F, whereas the multi-grade SAE 15W-40 is good down to about 10 F. The difference is on the "40" end, where a 15W-40 remains thicker (more viscosity) at operating temperatures than a 30-weight oil.

Your Cat manual should specify a temperature range for the oil choice. If Cat specifies SAE 30 for warmer climates, I'd be looking for a 5W-30 or 10W-30 for colder climates.  Those still work fine if it isn't cold - the oil acts like plain SAE 30 in warmer weather.  I would not choose any 40 weight oil IF Cat recommends 30 for normal operation, but 5W-30 or 10W30 is equivalent. 40 is not exactly harmful, but it is less than optimal if the engine is spec'ed for 30, and you should have no difficulty finding a API CJ4 type oil in 5W-30 or 10W-30. It's not as common an 15W-40, but still widely used. If your favorite store doesn't have it, shop elsewhere.  There is no need to accept a less-than-optimal grade of oil.  Tractor Supply Corp is one good source for various grades of oils.

All that said, I suspect if you look again at your Cat manual, you will find that it recommends different oil viscosity for different temperature ranges. 15W-40 may well be one of them. If so, using the 15W-40 will be fine.  You should choose the viscosity based on the temperature ranges you may encounter.

Thanks all for your input. I would like to point out that the 2004 C9 (an 8.8 L) engine is different from later 2007 C9 (9.3). Being said, is anyone with the older C9 engine using multi-grade oil?

Yes, and that's why you should go by what your engine manual says, not what others do with other engines.  But I would be shocked if Cat doesn't specify some multi-grade oils for it, and suspect they probably allow for an SAE 40 or 15W-40 in at least a few operating environments

I use Rotella t6 5W 40 in my Cat 3126

Thanks Greg, found engine manual and 10W-30 is preferred, 15W-40 for not below 15 degrees or up to 122 degrees

About what I expected.  So, 15W-40 is OK but overkill unless you expect to encounter very high temps. The downside of having a 40 instead of 30 is a slight loss of fuel economy due to the greater viscosity. Whether that would make any practical different in mpg is harder to assess.

CJ4 10w-30 should be readily available. Shell Rotella offers it, and so does Chevron Delo.  I know that Tractor Supply Company stores usually stock it, so try them if your local O'Reilly or Advance Discount store doesn't have it. Also NAPA Auto stores.  Heck, I've even seen it at my local Walmart!