Engine Oil Grade for Cummins

I have the same engines and have used Rotella 15-40 for the last 1800 hrs without any issues.

Grade can actually depend a bit also on your prevailing climate.
Ref. synthetic vs. mineral, I'd use the former only if and when recommended by the manufacturer.
Not sure about those Cummins, but for instance MAN prescribe synthetic for the CRM engines, but not for the older non-CRM.

We serviced our customers with NAPA 15W40 in Florida.
NAPA # 75121.
One customer uses shell synthetic blend 15W40 on his 8.3 HOs.

In Seattle, I would recommend not using a straight 40W oil.

Multigrade oil is much better for cold start, particularly in cold climates.

When I repowered my boat with Cummins 6CTAs they came with Valvoline 15w40. I ve now switched to Rotella 15W40 because that’s what I ve been using for years in anything from generators to 3412s and C32s

To thick on cold mornings. To thick for cold water.
10W30 may be a recommendation for colder climes.

There is also an API class rating on diesel engines. CH-4 is probably the minimum grade of oil required by Cummins.
You can not use gas engine oil in a diesel (API S*) only API C*.

Again, consult the MFG or the manual on this.

Don't confuse other post regarding Detroit Diesel engines oil specs. They require CF-2 only and that only comes in straight weights (30, 40 & 50Ws).

Gold in the garage. I hope big dawgs are there guarding these..

I found the Mobil distributor in N/E FL years ago and have been doing business with them directly since for our MAN customers.
If you have any kind of company credentials (Fed ID and tax number) or a friend with, I'd call your local Mobil distributor directly.

My partner has luck shopping the internet. Hit & miss but shopping and stocking has save him big bux. Sometimes free shipping with a min order.
Freaking cheaper than jobber cost sometimes.