I've got 3s and a 4 with overflow bottles. I check them every time before I start it up.
When I first bought the boat, it would burp a lot of what was in the drive bottles. The bottles even have a fill line on them. Probably for different drives. Anyway, after buying a gallon or so of Merc HP drive oil, and cleaning my bilge many times, I finally landed a service manual for the ssm 3, 4, and 5. Come to find out, there's not supposed to be any oil in the bottle when the drives are cold. On these drives, the bottle is more of an expansion bottle. Sometimes it is good to read the instructions....
I change the fluid at the end of the season.