YES! You can use Hydraulic/Transmission Fluid in place of 80/90 gear oil in a transmission.
The manufacturer recommends DOT 3 Fluid.
A. Your fuel system has an air leak.
1st - Check all the banjo bolt fittings for tightness and the hoses for cracks.
2nd - Pressurize the fuel system with 30 P.S.I. of air and use soapy water to check for any leaks.
Use 10w hyrdraulic oil or 10x hydraulic/transmission fluid.