Brian - I have a '54 and I believe it to be accurate that all Cubs (if not all, definitely the earier years from what I'm reading) have a set screw with a recessed allen-head between the hydraulic lines on the left side of the block just 1 or 2 o'clock to the carb as if the carb was "center to the clock." Remove the set screw and use either a really small funnel or oil
can to fill that reservoir up to the hole with 30 weight. It does the same thing as going through the filter and if you ask me, was easier since the hood is not obstructing the view. A couple of the guys here taught me that trick!