If you are running Castrol RS, when hot, this oil expands significantly.
This could be giving you a "false overfill".
The oil only gets to a significant high temperature when the engine is worked hard, and has no real relationship with the coolant temperature.
The excess oil will quite possibly find its way to obstructing the breather system. If this occurs, oil will be forced through all of the oil seals.

Check oil level after 5 laps
Fit an open breather system to a catch tank (1 litre)