I'm assuming that you raise the motor all the way up to relieve pressure and then open the screw and add the fluid. Is this correct? Do you bleed the system by leaving the screw open and lowering the motor?
Re: How do you add fluid to tilt/trim on Yamaha 50hp?
transmission fluid,dexronIII, is an acceptable subsitute. DO NOT lower that motor with the resevoir plug out. system is self bleeding as per the service manual.
its also a sealed system that will only require fluid if there is a leak.
if oil can leak out then water can leak in and the system wont tolerate water long at all.