What's your preferred diesel additive? Until now I've always used Flashlube but it's been suggested by a few very knowledgable people that I should be using Chemtech diesel additive instead. I don't know anything about the chemistry of those products but mostly the need for using additives in older diesel engines is due to changes in fuels and older motors not having the ECU's, etc. of modern mostly common-rail diesel motors for 'modulating' their operation to suit varying performance parameters.