I have a '70 commando with that same oil tank, and I don't see where you've added the magnafine filter, but I would like to see where you are putting it... And yes, the small higher up hole on the tank is the oil return side of the flow. I bought a magnafine filter over a year ago but didn't want to add it without securing it solidly because once it fills with oil, it will be heavy and bounce around if it's not secured properly.