The guys over at the ecj5 forums recommended using the 15w40 rotella for the old Buick motor in my cj5. They specifically mentioned the higher zinc content since the oil is meant for a diesel motor.
FYI, diesel oils have recently been changed to reduce zinc and phosphorus content. I run Delo 400 15W-40 in my F250 and have used oil analyses the reflect the reduction over the past few years.
But a diesel oil is still a good choice.
