I read and hear very often that a SAE 10W fork oil is recommended, when changing the GTs fork oil.
But you should know that a 10W from one manufacturer doesn't even come close to that of another manufacturer.
Important when comparing fork oil is the kinematic viscosity in Centistroke cSt or mm² / s at 40 ° C or 100°F.
If you compare oils under this information, you can see very large differences.
For example:
Castrol Mineral fork oil 10W is indicated with 32 cSt, Castrol Synthetic fork oil 10W with 42 cSt, the latter is almost 10% heavier, despite the same SAE and 42 cSt is also the viscosity of a Liqui Moly Motorbike Fork Oil 15W heavy Synthetic.
You can see that you should not rely on the SAE number, but always look in the data sheet of the respective oil.
By the way, a fork oil with about 22-25 cSt has turned out to be ideal for me.
Jerry