The main issue in transmissions is the "yellow" metal issue. You need to use a transmission lube that is compatible with the yellow metals it contains. GL-4 is considered safe, some GL-5 lubes claim to be safe, but I have my concerns and would stick with the GL-4. Mulit-viscosity lubes are fine (better in my opinion).
Not sure if the source you are looking at is using the older ISO VG rating or not, but if it is 160 - 260 would equate to around SAE 90, which is about right. To some extent it depends on how you like the shifting qualities, the viscosity of the transmission fluid changes the way the transmission "feels" when shifting it.