OK, the employee at the hobby shop is and idiot and you can tell him I said so. He's just trying to sell you what he has on the shelf to make a sale. Traxxas doesn't want you to run it in their nitro boats so that ought to give you a clue as the oil content is way too low. You want something with very close to 20% oil content. Even Heli fuel will work in a pinch.
Try MC-9's, not the 59's.
From the factory the low-speed needle is rich so hopefully the original owner took care of the problem. The problem with factory settings is that they don't know where in the world that the boats are going so they give you a rough guesstimate.
A prop that does wonders on the boat is an Octura M-445 or Zippkits 444. The Zippkits prop is ready to bolt on, the Octura needs to be worked on.
There should be some threads on this boat if you use the search on this site, tons of info and tricks, most at low to no cost to make it better.
Use only one head shim when you put it back together. You can also port match the header as it really chokes off the exhaust.