If oil system volume is decreasing you should see an increase in temperature as the available oil is spending less time cooling down before it's heated up again.
???? Just the same as it spends less time heating up again? Common misconception and wives tale.
Low volume where the oil reservoir is integral i.e. a gearbox will actually run cooler as the oil visits the cooler more in a given time period.
Bell 206 overfilled due to people misunderstanding the correct level in the MGB is a classic example.