we dont get all that much traffic here these days, but i have not seen any posts on one of these drives (not nearly as common as Mercruiser)
it looks like sierra still sells it,
from AI
"Type C gear oil is a unique, thin-viscosity lubricant primarily used in older outboards and stern drives with electric or hydro-electric shift systems (e.g., vintage Johnson and Evinrude motors)."
sorry that all i got, Joe