1996 Buick Park Avenue There is engine oil in the engine co

Also coolant appears to be leaking on the outside of the upper intake manifold. But only when the engine is warming up and the radiator is full. The leak appears to be on the rear of the car, side, of the manifold, and the throttle body end of the manifold.
Engine seems in good shape. no smoking, getting 26mpg on highway. Engine used about a 1/2 aquart of oil in last couple hundred miles. I had changed coolant and engine oil 1500miles ago so thats how I know theres oil in coolant and oil level went down. Starts great runs great.