Hey Bill - We use Rotella 15w40 oil for the saildrive. The fluid we saw definitely has an oily base. Color is yellowish. Does that help?
Hi... Your description of the fluid having a "buttery" coating sounds like water with an emulsion of oil/water floating on top..
I would think an oil/water emulsion couldn't just occur spontaneously but requires to be 'whipped-up' somehow, either within the engine or saildrive, as you might see inside a radiator cap or oil filler cap on a car with a blown head where some mixing of the oil and coolant has occurred...
Have a look inside the coolant/engine oil/saildrive filler caps and run your finger around under the rim to see if there's any 'mayonnaise' lurking..
If not, I don't think the buttery material came from the engine or saildrive..........Good luck.....Bill.