Which oil grade is approved for use in the engines of the CRJ900?

The approval of MIL-L-23699 (Type II) oil for use in the engines of the CRJ900 is based on its specifications and performance requirements that align with the operational demands of the aircraft's engines. This particular oil grade is designed to meet the thermal stability, lubricating properties, and other characteristics necessary for high-performance jet engines. It ensures that the engine can operate efficiently while maintaining protection under various temperature and pressure conditions encountered during flight.

In contrast, the other options are either not designed specifically for aviation use or do not meet the stringent requirements set forth by the engine manufacturers for the CRJ900. For instance, SAE 10W-30 is a standard automotive oil which lacks the necessary certifications and performance standards required for aircraft engines. API SN is a classification for automotive engine oils that also does not address the specific needs of jet engines. Lastly, Castrol 20W-50 is another automotive oil that does not align with aviation standards required for the CRJ900 engines. Thus, MIL-L-23699 (Type II) is the only choice specifically approved for safe and effective use in this aircraft.