The manual for my 304 calls for Chinese gear oil, which translates to 80W90 gear oil in US designation. The hydraulics call for AW32 or AW46 hydraulic oil, depending on temperatures encountered. I'm in the tropics so I use the AW46 with good results. The engine gets 15W40 premium grade diesel engine oil.
Using hydraulic oil in the gear case would be a big mistake – it simply is not compounded to stick to the gears, which is necessary in these straight-cut, splash-lubed gearboxes. Use the good gear oil.
All mechanical contrivances that need lubrication will perform better and live much longer useful lives if lubes with the right stuff, and have the lube changed on an appropriate interval. To do anything else is tantamount to throwing away the investment you made initially. Kinda like eating junk food and smoking for your body.