There may be some other factors at play:
- Not just density of water and oil, but one may need to consider emulsified oil/water as a third player. Mine has been like grease and sinks under water.
- the oil reservoir tank being about 1 foot above the water line (at least on my SN and likely your 54) gives the oil a "head" vs the water, so at the lip seal that would offset the density difference to some degree. Picture a ten foot tall pipe full of oil immersed in a bucket of water; even though the oil is less dense it will "leak" out into the water.
- the wipers on the seals are designed to microscopically "pump" the fluid being retained back where it came from. That's why your drive shaft into the top of the "C" drive isn't oily. At the same time, if there's fluid on the outside, the seal will tend to "pump" it to the inside. This, if I recall, is the crux of José Venegas' recommendation for water-water-oil. That is, keep "pumping" water away from the oil, which does allow a miniscule amount of oil through before the outside oil-facing seal "pumps" it all back, lubricating all 3 seals to prevent scoring of the bronze wearing-out-bearing.
As to "losing oil", I think if the seals were really shot it could be possible (and you'd likely see a slick around the boat). More likely, if the reservoir is down it's either a normal temperature expansion/contraction or, perhaps an initial underfill with some air working its way out. More likely, it is "gaining oil" as water is "pumped" into the lower unit.
Finally, can only guess, but seeing a small bit of "milk" after changing lip seals could, I suppose, be an initial "seating" of the seals, but perhaps more likely it is just some emulsified water/oil getting flushed free by the fresh oil that was added. That's why I rinse everything with diesel fuel, run the engine in gear and drain a couple of times before adding fresh oil.
That being said, I have hauled every three years for 20 years and always had some milkyness at the end. I'm curious to see if having adopted José's water-water-oil last March will keep me milk free - so far so good.
SN68 Sangaris, Tropic Isle Harbor, FL