Once you know the engine (e.g., 3.0L V6, 3.8L V6), consult your owner's manual for the *exact* specifications. The manual will specify the recommended viscosity (e.g., 5W-30, 10W-30) and the type of oil (conventional, synthetic blend, or full synthetic).
However, as a general guideline for 1999 Dodge Caravans, 5W-30 is a common and often recommended viscosity. But again, always check your owner's manual for the definitive answer. Using the wrong oil can damage your engine.