1995 Nissan Pathfinder Oil leak and low fuel efficiency

Engine problem
1995 Nissan Pathfinder 6 cyl Four Wheel Drive Automatic 149000 miles
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I have a 1995 Nissan Pathfinder, about 149,000 miles old. My mechanic told me that there is a major leak in the head valve gasket, but I dont find any oil puddles or stains where I park my car, and lately the car has been returning very low fuel average, approximately 100 miles in 10 gallons. What could be the reason for the low mileage.. is it related to the leak or is it an entirely different problem??

Thanks