Left the petcock open on my 1980 XLS and filled the cylinders with fuel. Pulled the plugs out and turned it over to blow out gas. put a cap full of oil in each and turned over to re-lube rings etc. changed the oil and filter and thought I should be set. Fired it up and noticed oil blowing out of Primary vent. Checked the oil level in primary and oil/gas poured out if screw hole. about 4 ounces over filled and has fuel mixed in it. How did fuel get in there???. this is an 1980 so no vent between crankcase and primary. A lot of gas poured down the outside of primary, could it have gone in through the shifter seal?? (seal is bad, One of the last leaks I have to deal with.)..... Any thoughts on this???