Blowing some black smoke

Drivers, my DD60 was blowing some fine black smoke this morning. I was accelerating through the lower gears up a hill, 40K in the box. I get around 7-71/2mpg average, so I'm running OK, but I'd like to get rid of that smoke. It's an '08 rig, has the DPF. Should I do some tuning? Adjust valves, clean injectors? Has new filters all around, uses zero engine oil in 15K miles. On the injector issue, is there an additive that's any good, or is that stuff all snake oil? Thanks.

 

Snowwy, thanks for this. I'll get it checked, at the Freightliner dealer in Albuquerque, where I bought it, when I'm on home call next time.

 

Skunk, I will check. I got a Series B maintenance as part of the purchase agreement, and I think they changed all the filters. I'll sure check this out. It would be a nice affordable improvement. I also think I'll still get my valves adjusted and injectors cleaned. Thanks for the response.