Dexron 3 is far, far away from being equivalent to SHC 629, he lied to you.
its a totaly different oil for a different aplication
Dexron 3 viscosity at 40 celsius is around: 35.9, Mobil SHC 629 is 150 mm²/s
at 100 around : 7.4, Mobil SHC 629 is 21 mm²/s...