What'S the best gas soluble oil to prevent rust?

If you are refering to inside the tank, the best way to keep it from rusting is to add fuel stabilizer, and fill the tank right to the top with fresh gas. Run the bike for 10-15 minutes to get treated fuel in the carb to prevent varnish. Then make sure the fuel valve is closed. Pour a spoon full of oil down the cylinder, through the spark plug hole, and slowly turn the engine over to coat the cylinder with oil, this will keep the cylinder and rings from rusting. Or you can just use an engine foging oil for the cylinder. To protect the outside surface of the tank, just give it a good coat of wax.

The fogging oil is by far the best.
Do the stabil like posted above and run it through the motor.
Then spray the fogging oil in through the carb so that its also through the motor and shut it down.
Normally this will cause a fouled plug in the spring, but plugs are cheap and a well protected motor will save you a good deal of cash.