which grade gear oil? I heard running gear oil destroys the syncro's in the t5's?

i use penrite sin75 its a 75w 90 gear oil.

i dont know about gear oil destroying syncro's,maybe a high viscosity mineral gear oil could?
the v8 t5's in eb/el run gear oil from factory...for some reason the 6 cyl ran auto fluid?
i know some gearbox shops wont give warranty on reco t5's if auto fluid is used instead of gear oil..

the ea box in my ed has close to 400.000km on it now and started making input shaft noise at idle about 50,000km ago, i started running the synthetic gear oil and the noise has gone, it does have a slight whine in 2nd but i would expect that with the amount ok kms it has done..

think mines beyond that, was doing 110 and heard a bolt ping around. stopped quickly to check it out and when i restarted the grinding started.