I have had questions about low ash oil for diesels. 

While ash content is now limited on 2007 and newer diesels, it was not in previous times. In fact ash content relates directly to the level of ZDDP, anti wear and TBN additive content/level. Higher ash oils are better able to provide wear protection and raise the TBN number to allow the oil to be safely used for a longer time. Older, traditional AMSOIL diesel oils, AME and PCO have an ash content of about 1.4%and a TBN of 12+, while the new DEO and DME have an ash content of less than 1% and a TBN of 10.

The old Detroit Diesel 2 cycle engines, like the 6V-72 and others having a “V” in their designation, called for low ash oil (below 1%). In that case the new DEO and/or DME would be preferred. 

DEO AMSOIL Premium API CJ-4 Synthetic 5W-40 Diesel Oil   

DME Premium API CJ-4 Synthetic 15W-40 Diesel Oil