We don't know what kind of or what year truck/car you're asking
about but I don't believe the synthetic oil is going to hurt
anything internal. I assume what you've read about is the
damage to the "brass" in a manual transmission. Those concerns
are with the high sulfer content of some Hypoid greases.
The one problem you may run into unless the trans is designed to
use synthetics is with the syncros. The syncro rings may not
grab as good with the synthetic, which can result in clashing
of the gears. This is especially bad whith the SM420. I suspect
it's because its gears have more mass than a 3 speed or the late
model 4 and 5 speed transmissions.
Give it a try, if it causes a problem shifting, drain it out and
refill, all you've lost is the cost of the synthetic.
Denny G
