I had carb issues with the stock 38 mm flatslide CV Mikuni carbs on my Ducati, and borrowed an original Suzuki synch tool from a HD mechanic just down the street. I believe it was the type with balls inside a tapered glass tube. This sort of worked OK, but the bike never ran perfectly at just above idle. So I used some clear tubing and ATF, and found that they still needed adjustment. After that, all was fine.
My point here, is that Hg (mercury) has 16 times the density of ATF oil, resulting in less resolution. Some BMW injected boxer owners have also found that using ATF in clear hose gives the required precision to get the desired result. So that is what I stick to when synching carbs.