nippodriv

Hi all...like so many others, I have a help question...

I have a 5-spd 2WD '90 truck, (many miles) and the transmission whines in forward gears 1,2,3 & 5, as well as Neutral, when the clutch pedal is in the released position (clutch engaged).

I replaced the clutch almost 2 years ago (it was still barely worn!) If I remember right, I got confirmation that the throw-out bearing doesn't have a problem, or I replaced it.

A relative who's a mechanic told me that mixing synthetic and non-synthetic gear oil can sometimes cause this. Also, a poster in the thread "Tranny Problems...... Maybe?" was very explicit about using only GL-4, and not GL-5 --why?

In any case, since it seems common, I thought it might help to ask what is it exactly that causes this whine? Is it simply shaft gears meshing with ring gears in a "loose" way? Or is it other worn parts like synchronizers or bearings, or an erosion of part surfaces from a bad oil mix? Assuming the tools are at hand, is it a decent project to tear down and fix, or should the whole transmission be replaced?

Thanks to any of you that can answer.