Well I am just about to hit 10,000 mi, and will be doing my second oil change. First time I went with Valvoline Premium Blue...good enough for Cummins good enough for me. The thing is the winter is coming and I got a shiny new plow on my truck. So therefore the truck will get a bit of a workout. When is too soon to switch to synthetic...any recomendations on brands. I have a friend that is an amsoil rep...seen that alot on here. Also anyone running fuel addatives in the winter, I've seen Fleetguard offers one.

 

I made the switch at 7000miles. I didn't go full on synthetic. Instead I mixed 2 gallons of Mobil 1 truck/suv 5w40 with 1 gallon of Mobil Delvac 15w40... I'll see what this gives me on my next oil analysis at 10k.

 

cummins recommends waiting until 15,000 to 20,000 miles. unless the truck has worked hard. it is on the cummins website.

 

I am waiting til 20,000. Thats what cummins told me, and I want the engine to be broke in well for good fuel mileage. I will be using amsoil.

 

Went to syn.at 15k

 

I would agree with Cummins recommendations, however, I believe they might be pointed toward a more commercial use of these engines. More specifically, a vehicle that might travel that 20k miles in it's first few months of use.
I feel, especially with cold weather, using synthetic will do no harm and only good. But to be on the safe side, mixing 2/3 synthetic and 1/3 dino oil, will protect break-in and cold weather use (to some extent). I always tend to over-think things.... jmo....

 

Just replaced all of my fluids with Amsoil Engine, manual Trans, X-fer case and both axles. seeing a 3 MPG increase in town and Hwy.

 

I'll be interested to see what you find.
I was thinking in similar terms, but more like going 1/3 syn blend my own. I haven't read of people doing this on their CTD, but from what I understand even a moderate amount of syn in there gives an improvement at temp extremes.