It is a common practice to run a light weight oil instead of 10w30 engine oil. It makes it shift better.
Hydraulic Fluid I dont know about. I do know that there is some equipment (tractors, loaders) that run the same fluid for there transmissions, and the hydraulic system. It is usually sold in 5 gallon buckets. What are you going to do with the other 4.5 gallons?
You can buy a light weight oil that is made for 2 stroke transmissions. It will be wet clutch safe.
Try bel ray gear saver 75w make sure you get the right oil. Bel Ray has several different gear savers. You dont want one that is 80w-90 that is a hypoid gear lube made for differentials. The ones you want will just be 75w, 80w, or 85w.