very common on that series of engine. Briggs and scrapiron I mean stratton. VERY common! The oil pan-aka crankcase cover-gasket is leaking. Generally they get "sucked in". Once you pull the pan you'll see it (engine removal required).
There is a kit to "fix" it. It's a gasket and crankcase cover bolts. They tell you to replace the gasket, the crankshaft seal, and the crankcase cover bolts. When replacing the bolts they want you to torque them to 300 lb-in as I remember. They are 5/16" bolts and 300 lb-in is pretty high for a 5/16 bolt. Some of them I have had them pull the threads out of the block and have had to repair, not common but it happened. With that said you will want to absolutely use a good calibrated torque wrench, and use no lubricant on the bolts (they come pre-coated with a locking agent).
Once you're done there you get the fun of installing the crankshaft seal. Ideally it would need a special tool but I have done it before with a plastic bag and/or a set of feeler gauges. It ain't no fun for sure and once you get the engine upside down sitting on the work bench you will see what I mean. Briggs and scrapiron I mean stratton machined a 90 degree step in the crankshaft and when installling the seal the step in the crank aligns perfectly with the step in the crankCASE, such that without some "help" (tool) the seal cannot be installed properly.
Last piece of advice I have is to go ahead and remove the oil pump cover, the pump gears, and the pump drive shaft before removing the crankcase cover. Nearly impossible to put the pump drive back into the cover and it stay there while you're trying to reassemble the engine, so just pull it out ahead of time, and install it last. MUCH easier.
Governor linkage can be a pain in the neck too. Make sure to note how everything came apart and which way the gov shaft has to be turned. If you get it off, the engine will not maintain RPM, it will run about 6000 RPM based on experience before you can get the key shut off. My coworker actually did this (well so did I but I caught it quicker), and on his he kicked a rod out which destroyed the engine. Also be careful with the governor gear in the engine while assembling the engine as the plastic gear can be easily damaged. If the cover doesn't fall back down all the way on it's own, rotate the crankshaft a little by hand until it does.
"Fix" meaning if you do it correctly you may get another 400 hours out of it. If you don't have a torque wrench and/or don't get ALL of the old gasket off, you'll be pulling the engine again in about 80-100 hours with another failed gasket.