My old original mercury filled Carb Stix has a central reservoir shared by all 4 tubes. I'm thinking of building an oil one along the same lines. I've never had any problems using the mercury gauge other than occasionally the columns will show air bubbles in them. All you do is unplug the hose so the column drops back down into the reservoir then plug it back on the carb so it takes a fresh pull of mercury. I think it's pretty accurate. The slightest adjustment on the sync screw changes the column heights. With oil or mercury, the goal is the same - make the columns even or equal.