Has anyone used Amsoil synthetic Transmission lube 75W-90 GL-4 in their 1969 4 speed?

hi, well, have to comment here. with heavy gear oil, it will cover up a lot of sins in the transmission. years ago, car lots put the thickest gear oil and stp in manual trans to sell a car. trans sounded fine, till the oil was changed, then heard whining, gear clash, grinding in trans. usually they blamed the oil for the problem!!! 140 gear oil does not lube till it gets hot!!!! therefore, the trans runs dry!!! this causes wear!!! a lighter oil will lube always. ATF is ok, but for short use. the synthetic gear oils are superiour to std oils!!
i have ran manual trans for last 35 yrs in my street and drag cars. my 833 would not have made well over 3000 passes with regular 140 oil in it!!
I inspect it about every 500 runs, there is no wear on countershaftor bearing areas. the gears look like new!!! synthetic gear oil is only way to go.
and btw, the 833 is in a mid 11 sec duster, it gets pounded on very regularly.