Just curious where you saw the "GL-5" requirement, is that in the owner's manual?It needs the GL-5 gear oil for types of gears in it from what I can see. There is very little oil actually in it. I am guessing most all the oil is coating the gears when it is running and the gears are not really running through the oil. Just get GL-5 rated is the most important. Could try this stuff. 75W85 GL-5 Gear Oil
Thank you, I was unaware of those tech .PDFspage 32, "SAE 75w-90 gear oil." The GL5 info was from Forum member in contact with Stark support, as I remember it..
07.019.06 CHANGE GEAR OIL PDF, Under Tutorial & Technical says, "Synthetic gear oil 75W-90"