As mentioned before, my own ARO is a 50/50 mix of ATF and Air Tool Oil, (basically turbine oil). Been using this for 30 years now. Some fuels with synthetic lube seem to work best in preventing corrosion, but I agree with Duke Fox that at least some should be our tried and true castor. Much better at maintaining lube properties under the high temps 4-strokes are known for and any lean run or low airflow, (re. bad cowl installation) where synthetics burn off too quickly to do the job.
Bill, that fitting on the back or bottom is the Case Vent, used to balance the volume change as the piston travels up and down and to allow blow by gasses, the excess lube and other contaminants to clear. One piece of advise on that; If you attach a hose try and keep it as short and/or as large as possible to prevent negative pressure from reingesting any crud before it makes it out the end.