I haven't driven a mani for decades , and was waiting for someone to give you a headsup .
I had a sluggish gearbox in a car yonks ago and thought I was up for a rebuild .
Someone suggested a different grade synthetic oil [ amsoil , molyslip , ? ] - I can't remember now twas long time ago .
Was like driving a new g/box . Smooth . quick and quiet .
Could this be your answer ?
Just a thought .
Someone who's not a farmer like me will be along soon .