5w30
If you will follow the top radiator hose to the engine, you will see were it clamps to a housing. It is under that housing.
I've got a better question, why is this question in the Chevy venture forum ?
Lower ball joints wear out early. Door hinge pins and bushings wear out early. Idler arm and pitman arm joints wear out early.
You did not mention the engine size. Regardless, it is in the thermostat housing on the intake manifold.