2000 cadillac deville

why would a person change oil for this car from 10/30 sae to 15/40 diesel oil ?

1995 Cadillac Deville

car has been dying after driving a few miles always starts right back up only to go aways and die again. help!!!

2004 cadillac deville

I have no power to the car checked battery and fuses both are good what else can be the cause