PC-23 Technician reported installing an instrument cluster that was not effective for the aircraft identified.

Narrative

After receiving a discrepancy list from one of our line pilots describing the legacy resistance-type oil temperature gauges as inoperative I proceeded to inspect the system. In a continued effort to increase Dispatch reliability for our aircraft; when possible; the general tendency of the maintenance organization is to deactivate legacy gauges and instruments; when legal to do so; in accordance with FAR part 43; and 91. In this situation; I erroneously believed a previously installed engine monitor to be sufficient (certificated and STC'd as a primary instrument replacement for oil temp). After calling my immediate supervisor on DATE to confirm my initial findings he confirmed to me that the [previously installed motor] would be sufficient and I proceeded to deactivate and label the legacy oil temp gauges as inoperative according to 91.213(3)(ii). Fast forward to DATE1 and another discrepancy was reported concerning the legacy resistance type CHT gauges. After vaguely remembering my process for deactivating and placarding the oil temp gauges inop; I started to do my due diligence on the CHT gauges as well. This led me to the flight manual supplement for the JPI 760; which I must have either read incorrectly or omitted to reference back in April; in which I found the operating limitations stating that the gauge cannot be used in lieu of or for replacement of primary engine gauges. After calling my supervisor to explain my discovery and realizing the mistake from back in April; I proceeded to diagnose and repair both the Oil temp and CHT legacy gauges. A logbook entry was drafted and signed on DATE1. stating the extent of the work performed and mailed out the same day to maintenance headquarters to be added to the aircraft permanent records. The aircraft is now legal according to the aircraft POH/Flight Manual and 91.205.