Hi Brian. I'll go along with the gang and say "change it". Do you have a vacuum oil extracter and spare oil? If you do it's a fairly quick and easy job.
No need to panic however, the fact that you have milky oil means that it is emulsifying water. In the meantime, running the gearbox
regularly will keep the water emulsified, so that's better than letting it sit.
I'm a bit skeptical about saildrive
oil header tanks
. They may be fine on some models but not others. The Yanmar
SD50s; that well documented piece of crap, were originally sold with an incorrectly high oil level. This caused input shaft oil seals to blow out due to oil pressure being too high. The solution; replace oil seals and retrofit a new longer oil dip stick, so less oil and more air space above the oil. This happened to our SD50s in the first few hundred hours. Fit an oil header tank to a SD50
and this problem will no doubt reappear.
I hope your SD60s are an improvement.