We all forget these motors are made by Cummins. What do they say? Whatever is in your owners manual, that is what you should do. They say nothing about syn, at least in Gen 2, and I think not in gens later either. They do endorse Valvoline 15w40 PB, as it does have the highest TBN numbers, at leat in CJ, as well CI+4.
This is reality to me. The rest is marketing. In reality, changing oil on schedule and sticking with a quality oil, delo, rotella, Valvoline, etc. is best practice. Just ask Cummins.
My $02.

 

4th Gen owners manual does say you can use synthetic lubricants. TBN is only a portion of what makes an oil a quality oil. The Valvoline Oil mentioned doesn't have the highest TBN either. The Oil Analysis I had done on the AMSOIL CJ-4 5W-40 (10.4 TBN) with just under 18K miles proves you can go longer than the OEM recommended drain interval. Granted my truck is deleted so I don't have to worry about dilution and I would never do it without routine oil analysis if everything was still intact. On Monday I will be taking a sample on the AMSOIL CI-4 15W-40 I switched over to and will post the results once I get them back.

 

I started using one of these about six months ago and have compared it to oil analysis.
It's gotten to the point where I'm going to fully trust it for oil change frequency> http://lubricheck.com/

 

We run john deere plus 50II and polar/total power with no trouble We've got tractors with 40000 hours and nothing done to them but a valve adjustment

 

We figured 35miles to an hour of runtime

 

I finally got back the results for the oil sample. All is good, my observations are below.

Oil Sampled: AMSOIL AME 15W-40 Synthetic Heavy Duty Diesel & Marine Oil (CI-4+) - 4,245 miles on oil; 49,249 on truck

1. The analyst mentioned that the magnesium was slightly high for this oil. I may call AMSOIL to see what they say. But I think it may be elevated because of the additive package used in the AMSOIL DEO 5W-40 CJ-4 oil (that I replaced) contains a lot of magnesium and may have just been residual that was left over from the oil change.

2. Aluminum was slightly elevated compared to the last sample and the potassium was down which means coolant is more than likely not the source. All of the other wear, multi-source, and contaminate metals were down compared to the last oil sample.

07JAN2013 Oil Sample