API GL-5 is a formulation specification that controls the additive performance required to protect the gearing at startup and under shock loading.
220 is a viscosity in the ISO's VG viscosity grade system. 90w is a viscosity in the SAE viscosity system. It doesn't matter which one the lubricant measures at as long as it's the right viscosity in either system.
Both the formulation specification and viscosity matter. Does the handbook say anything else?
If not, the parts store probably sells a lot of multi-viscosity GL-5 gear oils such as a 75w-90, 85w-140, etc. Not sure offhand what's popular in a tractor parts store. I would then contact Ferri and find out 1) which multi-grade viscosity is appropriate and 2) if API GL-5 gear oils are appropriate.
Worst case you may need to find a couple of quarts of industrial gear oil (MSC, Grainger, local industrial lube shops) such as Exxon Mobil 600XP 220. That should meet the specs.
In all cases you're playing in the correct field and although there might be more perfect lubricant selections out there, you aren't proposing anything that's simply wrong. Personally I'd put some synthetic industrial gear oil in there for a few extra dollars and take comfort knowing that my flail gearbox is running as cool as possible with maximum protection.
David
