This happened to me after 5000 miles on my last rebuild, and I have used
MTL long before that rebuild and continue to.  I now have over 100,000
on my last rebuild.

I called AA Transaxle and Craig, who ran the place at that time, said
bring it back in.  It turned out the pinion bearing was improperly
hardened and started to fail quickly.  I had to drop the box again, but
the repair was covered under warranty.

Take it back to the guy who did the job and see if he will honor his
warranty. I hope you have one, if the noise you describe is proportional
to road speed only a bad pinion bearing, differential carrier bearings,
or output shaft bearings can cause this.

You could try Red Line MT-90 next time, but shifting will be harder.
Don't blame the oil for this noise, something is not right inside your
tranny and it needs attention.

Stuart