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Well, you could do a vacuum test as well, but that was never recommended by OMC for these style gearcases…Until later years when “back to back” seals were used. One set of seal keeps oil from pushing out, the other set keeps water from pulling in… So, using a vacuum tester might just overcome the seals natural resistance and add even more confusion…
Like others have said, it is always a help to do the pressure test in a bucket of clean water. It is not enough to just add pressure and watch the gage, you really need to move the shift shaft up and down, rotate the shafts while pulling them in and out… Doing all this while submerged in the water will show any stray bubbles…And, like others have said, bump the pressure up to 10PSI or so to check condition of orings. Finally, are you sure this isn’t just an issue of a missing or damaged seal washer used on the fill drain screws? Sometimes the screws get swapped, so there are two gaskets on one screw, and none on the other.
In any event, the little bit of emulsion showing is not that big of a concern right now…I would not pull that thing apart until such time that you have found the actual leakage issue/problem or the leak gets much worse.
Change the gear lube more often next season just to monitor the situation…