Which oil for my 2003 C15!?

You can even sample coolant and the oils in your transmission and diffs if you were so inclined.

 

Excellent, I see a lot of new owners who just get the truck and that's it, they think it is a car.

By the way, when you do choose an oil, stick with it.

 

I worked at place with 120 pieces of heavy equipment and 30 trucks, Delo 15w40 in every engine.Never had any
problems.

 

I've always used Rotella, change every 15,000 sample EVERY change. Cheap insurance.
I did have to use delo for a change at the forst of covid, couldn't get Rotalla, and for the first week the stink tube had real weird caustic smell from it? It eventually went away but I figured it was something they had in their additive package? Anyway, back to rotella. Many years, 6 trucks and too many mil ees s, all Rotella. Samples always look great, when they start changing you can address things quickly. Dont do bench top, use a good lab, and keep using the same lab so you can compare samples.

 

Hah, yeah. Fresh Rotella does put out a distinctively pleasant odor from the blow-by tube compared to most of them. I like the T6 15W40 myself in the warmer months and the 5W40 in the colder ones. There are the crazy expensive oils that are a pain to find like Amsoil or Schaeffer and then there's the entry level stuff like T4, Mobil 1300, and Delo you can find on the shelf anywhere but T6 is the sweet spot IMO where price, performance and availability all look good.

 

100% on the CAT branded filters but I wouldn't waste my money or time until they are actually due. The oil is fine for awhile if it's only 6k on the OCI I'd just go out and run it. No reason to change brands either if the Delo shows a good UOA, which I would recommend doing when it's changed just for informational purposes.