Have 28 year old Yanmar 3GMC (the C indicates connected to saildrive) and SD20 saildrive. You, most likely, are talking about a SD20 saildrive.
For a number of years Yanmar has made a 2 piece zinc, which
you do not have to remove prop to install. Zincs also can be installed while boat is in water
. Cost for the 2 piece zinc runs
around $50. There is an additional zinc inside the saildrive upper
portion, inexpensive and easily installed depending on your individual access.
filter is on the side of the engine and easily accessed, again depends on your individial install.
Topic comes up regarding changing saildrive gear
oil, as the SD20 is only drained from the bottom of the saildrive leg.
As I am in the northeast US and haul every year this is not an issue for me. Know that many owners of saildrives do not change gear
oil every year, depending on usage.
Hope this helps.