[MetroMPG]

Did he say why he didn't recommend synth?

Synthetics actually make more sense in drivetrain components (vs. engine) because they don't benefit from the heat of combustion to warm the oil & allow it to flow more easily. Synth fluids have more stable viscosities in cold conditions than mineral oils.

If you live somewhere that experiences cold weather, I would say the lightest weight, synthetic oil you can safely use in the transmission is what to go for.

The installer by definition sees lots of gearboxes that are damaged by hard use and neglect, and that probably colours his lubrication recommendations.