Well the best I have found is maxmia MTL (Stands for manual transmission lube).

They make it in 3 weights, 75 which is lite, 80 which is regular, and 85 which is heavy. I'd run the 80wt regular (MTL-R).

Now if maxmia is to expensive run a good quality type-f tranny fluid.

What ever you run, change it often, every 10 to 15 hours to keep all the particles cleaned out of the motor.

If you shop for the maximum on amazon it's only about $1 to $2 a qt difference from a good quality type F fluid.

I would change it more often than every 10 to 15 hours.  With Suzuki's metallurgy I would drain it at the end of every weekend at the end of the last ride while it is hot.

If you wait until you get home most of the metal and clutch material will be stuck to the bottom of the cases.  When it is hot most of the particles are suspended in the oil and will leave with the hot oil when it is drained.