Oil in the fuel only insures your internals get lubricated. The air/fuel ratio for an internal combustion engine is ideal at 14.7:1. It's really hard to tell without a wideband O2 sensor. It's more of trial and error. If you have one of those IR temp gauge you can take a temp of the head right beside the spark plug and that will give you a indication of how lean it is, the hotter it gets, the leaner it is.