[1stDave]

I just completed the same procedure on my Z275. The problem seems to be rainwater entering the the filler cap.
When I parked my boat at home for winter it was tilted to starboard. Water filled the ledge under the engine hatch and overflowed onto the trim assembly. I used my Oil vacuum pump to suck out the bad oil-water mix.
Tested trim up and down about 10 cycles after refilling with fresh 10W30.
Oil got a little milky so I sucked that out and tried again. One the third time the oil stayed clear.
I'll make sure that the whole boat is covered this winter. I may also install drains at each corner so water goes directly into the bilge. (my boat is not covered in summer).
Just a thought.