The Wayback Machine - https://web.archive.org/web/20190529205946/https://www.ip-forum.eu/howto-fritz-box-firmware-images-auch-unsignierte-ueber-den-bootloader-installieren-577

The allowed before the sump hits its thermal threshold.

The document breaks down efficiency losses with granular detail:

Thinned oil fails to maintain an adequate hydrodynamic film, leading to metal-on-metal contact.

The advantage is : no testing is required. However, the accuracy is lower – as noted earlier, deviations of around 14 % from measured values are to be expected. The document explicitly states that these calculation methods are “to be regarded as an alternative method” and should be used when testing is impractical.