I use to be an electronics technician. One of the first things they taught us about troubleshooting is to check the last thing you did. You changed the oil 500 miles ago. Check your filter and drain plug to make sure they're tight. When you're cleaning up the oil, also look for the source. If you can't find the source, then fill the crankcase and run the engine, looking for the source while the engine is running. You may need to put a few miles on it or rev it up, but you should be able to find the leak.