What type of oil is recommended for the manual gearbox and drive shaft of most one-ton trucks?

The recommended oil for the manual gearbox and drive shaft of most one-ton trucks is SAE 80-90. This designation indicates a gear oil that is specifically formulated to provide the right level of viscosity and performance for heavy-duty applications. The SAE 80-90 oil has a viscosity rating that ensures adequate lubrication under high pressure and temperature conditions typical in gear systems, which is critical for the longevity and reliability of the truck's drivetrain components.

The multi-grade oils and synthetic options, while beneficial in other contexts, are less commonly specified for manual transmissions and differentials in heavy-duty trucks due to their intended use and properties. SAE 30 and SAE 10W-30 are single-grade and multi-grade oils, respectively, designed more for engine applications rather than gearboxes where the load and operating conditions require the characteristics found in heavier gear oils like SAE 80-90.