You have a VRO pump on that motor. There are quite a few places which oil may be coming from. Oil delivery hose to the engine, then to VRO pump. Could be a fuel leak also, the fuel evaporates and leaves the oil residue. You have recirculation hoses which tend to crack at the fittings over time. Your primer can leak fuel which evaporates. You need to check, and recheck all places where fuel and oil travel, even the carbs. If you have plastic carburetor bowls they may be warped and leaking. Remove the airbox for a better look. You may want to remove the lower cowls also and trace the residue you are seeing. You can run the engine with the lower cowls removed and the airbox removed. You need to be cautious as you do not know if it is just oil leaking or you have a fuel leak. Fuel leak can cause a fire, a bad running condition, , oil leak can cause a catastrophic engine failure. My guess is a fuel leak as it occurs after sitting in the sun the fuel expands and finds its way out . If it is an oil leak or a fuel leak it needs to be corrected before you run ,and possibly ruin that motor. Clean the motor so that you may possibly spot the spot. I use a parts cleaning brush, paint brush or something similar with lots of Dawn dish soap then spray it with a hose. Of course be attentive to your wiring(disconnect at the battery) connectors and your carburetors, not a good idea to spray water into them. There is no single answer to your question.Let us know what you find.