Buick La Crosse Engine Oil Leak

My Oil Was Unexpectedly Low (like 3.5 Liters Low). I Knew I Had A A Small Leak But Not That Bad Because There Hasn’t Been Any Oil Stains...

Engine Was Down A Quart Less Than A Week After An Oil Change. Filled The Oil, Drove Maybe 10 Miles And It Was Down Another Quart. Mechanic...