Many countries are moving from Euro III ( +500 ppm Sulphur) to Euro IV (>50 ppm S) and Euro V (<10 ppm S) with regards to the type of fuel / Diesel available.
Correlate the above info with the age/model of Diesel engine. The newest generation of engines ( along with DPF) would require CJ4 type of DEO, while for the older engines CI4 or CI4 would suffice.
The next query would be customers choice to move from existing Mineral oil to Semi synthetic or to full synthetics.
Having acquired the 3 basic info for a change, if the customers has any complaints or reservations.
It is only after you have ascertained the above info proceed to offer alternatives.