This might sound a little strange at first, but 10w30 engine oil is what is run in the hydros in these machines. Some guys run the same oil in everything, ie. chaincase, engine and hydro, thus having to only stock one variety of oil. Personally I run 10w30 in the hydro and chaincase, and 15w40 in the engine, due to the high temps we get around here in the summers.
Hope this helps some,