The new CJ-4 spec oils for the newer low emission diesels are in fact low ash oils. It's part of the required specs to be called CJ-4. The older CI-4 spec had higher ash.
Any of the newer 15-40 diesel oils that are ok for "low emission" (i.e. Rotella T triple protection or Delo 400 LE) are fine for gas engine use and are designed to be used in mixed fleets.
Tractor Vet is right in that the older diesel oils (CI-4) were high ash, but the new CJ-4 spec is low ash.