Which oil for my 2003 C15!?

I would do an OA, you should have done this before you purchased it, the OA should tell you the condition of some of the engine components and will give you a good baseline.

So how did it dyno out?

You meet the 85% of rated power?

 

To me its all the same if it meets Cat's specs.

 

Some people have preferences. I've heard it claimed that some oils have less blowby or less consumption than others.

My old man ran 15W-40 Delo in everything he owned - 3406A and MBN Cat, Series 60 and ISX with no problems at all.

 

Citgo C500 15w40, Mobil Delvac 1300 15w40, Delo 15w40, all very good Diesel engine oils, plenty of others ‘big name’ engine oils on the market.

 

I don't think you understand, you should always be concerned about the truck, and these things are part of picking out the truck right to make sure there isn't a lot of downtime.

You need to do Oil Analysis, it is actually recommended by Cat. It tells you when you should be changing the oil, right now you are in a good position to establish it with before your first oil change.

An OA can tell you a lot about the engine's condition, especially the bottom end.

Blowby is measured at a full load on a dyno, it tells you the condition of the rings. The dyno tells you the amount of HP the engine is producing and you compare that to the rating of the engine to tell you if the rebuild was any good or when it needs to be rebuilt.

As for oil, what others said, but as for cats, they are more demanding.

The game isn't to be a big-time truck driver with a big hooded truck on the side of the road but to be one driving down the road.