R22 MR TEMP indication system
The subject MR TEMP warning indication system must be operative.
Refer to 14 CFR § 91.213 Inoperative instruments and equipment.
...(d) Except for operations conducted in accordance with paragraph (a) or (c) of this section, a person may takeoff an aircraft in operations conducted under this part with inoperative instruments and equipment without an approved Minimum Equipment List provided—
...(2) The inoperative instruments and equipment are not—
(i) Part of the VFR-day type certification instruments and equipment prescribed in the applicable airworthiness regulations under which the aircraft was type certificated;
....
14 CFR 27 is the Certification Basis of the R22 (ref page 6 of Type Certification Data Sheet No. H10WE), and includes:
§ 27.1305 Powerplant instruments.
The following are the required powerplant instruments:
...(f) An oil temperature warning device to indicate when the temperature exceeds a safe value in each main rotor drive gearbox (including any gearboxes essential to rotor phasing) having an oil system independent of the engine oil system.
Also refer to Advisory Circular 91-67, Minimum Equipment Requirements For General Aviation Operations Under FAR Part 91 > Chapter 2. Conducting Operations Without An MEL.