For Isuzu NPR models from 2007 or later, a silicone-based transmission fluid, such as the Isuzu SCS automatic transmission fluid, is necessary. If the truck is older, look on the dipstick tube for the transmission fluid indications. The dipstick is on the driver's side when the cab is flipped forward.
If a silicone-based transmission fluid is not put into the newer Isuzu NPR models, the transmission could burn up quickly. This fluid is inexpensive, and it must not mix with other fluid. The newer NPR models that take this fluid have a vertical door handle, while the older models have horizontal handles.