The intake manifold gasket may develop and external engine oil or coolant leak. In some cases, an internal coolant leak may occur causing coolant to mix with the engine oil. Running the engine with a coolant/oil mix may result in engine damage. Replacing the intake manifold gasket should correct this issue.
Car Problem Reports
Chevrolet G Series Van (G10) Intake Manifold Gaskets May Leak Coolant or Engine Oil
Chevrolet G Series Van (G10) Problem
Engine Affected: 5.7L V8
Average Mileage: 135,000 mi (70,000 mi - 200,000 mi)
Visitor, 1996 Chevrolet G Series Van (G10), 70,000 mi
Coolant leak was found to be the intake gasket, The problem my be related to the Dex-cool extended life coolant that will become acidic and destroys the gasket. I would recomend changing the coolant at least every two years this will help prevent but it may still occur.