20:1 is crazy rich on oil. I'd use Redline oil, mixed 40:1. That works in pretty much everything. I'd also look at your spark-plug at this point. If the engine is spewing oil(likely due to rich oil content), the plug is most likely nearly fouled. That plug will tell you a story too. If will probably be wet/oily/black... which means too much oil. There's a good chance anyway.