I use 40w pennzoil platinum.

Seems to work fine.

Most will be ok so long as they don't have any friction modifiers and you change them regularly.

I used belray gear saver a loooong time ago and i got some surface rust as well as a bit of pitting.

Use a name brand 10w30 in the winter and 20w40 in the warmer weather and change it frequently.  You do not need to spend $8.00 to $20.00 per quart for the "special transmission oils". Guys that spend a lot of money on the special oils tend to not change it as frequently.  It is better to change it more frequently with a cheep oil and get the metallic flakes that are constantly increasing in concentration and circulating through the gears and bearing and accelerating the ware.  The metallic flakes come from the corners of the shift dogs.  A small flake is removed from the dogs with every gear shift. 

Heavy viscosity oils will offer a little more gear protection but clutch slippage may occur.  Do not use synthetics, STP, slick 50 or any of the so called friction reducers.........they may cause the clutch to slip.  ATF makes the clutches work well but may not offer the gear protection needed on the substandard metallurgy of the Suzuki transmissions when used for general riding where the oil temps are much higher than in a drag racer. 

Over 7 to 8 Bucks a for 32 oz bottle? I ran Type F trans fluid forever with no problems, I just went to standard 10-30 GTX and change it every other trip, I understand get it with some of you with these 70 HP plus engines using the High end oils in the gear box looking for that extra protection.