Most of the time it's the inside bearing on the output shaft . That bearing is located at the highest point in the trans. Because lubrication depends on gravity and the gears themselves to pick up and distribute oil, that bearing is prone to starvation, especially if the oil level is not maintained.
Over filling the case slightly by tipping the motor (a couple of tablespoons) won't hurt and gives that bearing a little help. Make sure the trans vent is clear and unrestricted. There may be some initial oil blow from the vent tube but that goes away as the oil level and internal pressure balances out.
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