Is it possible for oil to enter cooling system from a intake manifold leak ?

Yes it is very possible. Coolant Is Very Hard On The Rod And Main Bearings. Your Engine Maybe Ok But No Guarantee. Oil Passes Though The Coolant Section Of The Gasket, We Had A 03 Ford Cargo Van With Same Issue, Intake Manifold Failed At Oiling Port. We Did Replace The Gaskets And Did A few Oil, And Filter Changes Before The Customer Received There Vehicle Back. Hope That Helps