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The early Volvo manual boxes without OD units used gear oil. With the advent of the overdrive unit, a lighter oil was needed as the OD uses the same oil as the tranny and has a hydraulic pump and actuators. So multiweight motor oil was specified. With the later transmissions, different gears were used, and since the OD unit is essentially a small automatic tranny, ATF was specified. The ATF provides the correct friction characteristics for the OD clutches as well as the hydraulic properties needed to operate the clutches. And it still has the lubrication necessary for the gears in both the OD and the manual tranny sections.