What is the reason for this engine oil service procedure on a Bombardier CRJ?
The Aircraft Flight Manual or Flight Crew Operating Manual will specify the timeframe during which engine oil may be checked after engine shutdown. For the CF34-3B engine on a Challenger 605, checking must be done between 15 minutes and 2 hours following shutdown.
If done outside of this window, the engine must be dry-motored (cranked without introducing fuel) for at least 30 seconds.
Otherwise the oil level may (incorrectly) show too-low (the oil tank is not at the bottom of the engine), which could lead to overfilling. Overfilling is said to be worse for jet engines than operating them with slightly too-low oil levels.