Sorry I don't have an answer. But it is sad commentary for Mercury if it is this hard to find. A very basic spec. Which is why Honda is to be praised. Their service info for cars, bikes, OPE, everything, is easy to find to and very complete. Well written and illustrated manuals.
The availability of service info weighs greatly when I purchase.
Edit: I just googled this subject and found tons of info very quickly. How hard did you look?