Now those are good questions.
The oil we need to use in our Merc sport jets is Mercury (Quicksilver) "High Performance" gear lube. It's not cheap (about $12 qt) but it's the best for the job. If you want to try to save a few $$$ then you can use 90wt gear lube. (I do not recommend that)
You need about a quart for the gear case... and about half a quart for the tail cone. (you can see the fill plugs in the end of it)
The impeller isn't too hard to get out, as long as your pump isn't corroded. Also... it's highly advised to buy the mercury impeller nut tool.
And finally... I DO NOT recommend using any solvents in your fuel. It WILL destroy the oil film layer, and can cause piston scuffing. The only thing that should go into your fuel tank (other than fuel) is a good marine fuel stabilizer.