Here's why:
* Different Oil Cooler Systems: The 1990 truck likely has a simpler oil cooler system, or possibly no oil cooler at all. The 1996 engine probably has an integrated oil cooler system with its own lines and fittings. These are almost certainly incompatible.
* Block Differences: The engine block itself might have different port locations for oil cooler lines, even if the 1990 truck *did* have an oil cooler.
To successfully install the 1996 engine, you'll probably need to:
* Either: Remove the oil cooler lines from the 1996 engine and plug the ports appropriately. This is the most straightforward approach if the 1990 truck doesn't use an oil cooler.
* Or: Source oil cooler lines and fittings compatible with both the 1996 engine and the 1990 truck's oil cooler system (if it has one). This is more complex and requires careful attention to fitting compatibility and correct routing.
In short, while technically you *could* leave the lines on, it's impractical and unlikely to work without significant modification. You'll need to assess the oil cooler systems of both the donor engine and the recipient truck to determine the best course of action. Consult a repair manual for both the engine and the truck to understand the oiling systems thoroughly.
