When the oil level in your Briggs & Stratton engine seems to be perpetually low and the mower isn't smoking while it is running, then the seal between the engine block and oil pan is most likely bad. Briggs & Stratton engines typically burn some oil when the piston rings begin to go bad, but they usually don't go bad for a few years. Replacing the oil seal gasket takes a few special tools, but is well worth doing yourself to save the money you would otherwise have to pay a mechanic.
Unscrew the oil drain plug located on the base of the Briggs & Stratton engine block with the correct size wrench. Put the oil drain pan under it to capture all the oil. Screw the plug back in tightly.
Set the Briggs & Stratton engine on its top so the crankshaft is facing up. Fit the proper size socket wrench on the bolts screwed into the base of the oil pan and unscrew them.
Pull the oil pan off the crankshaft. Peel the old oil seal gasket off the perimeter of the oil pan. Wipe off any oil with a rag.
Place the new oil seal gasket on the oil pan. Slide the oil pan back on the crankshaft against the engine block.
Tighten the bolts back through the oil pan with the torque wrench. You have to tighten them down with 15ft-lbs to properly seal the oil seal gasket. Screw a bolt in on one side of the oil pan and then screw a bolt in on the opposite side of the oil pan. This will evenly tighten the oil pan down.