§ 91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.
(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
(5) Oil pressure gauge for each engine using pressure system.
What is a "pressure system" in this regard?
Background: Our aircraft has an oil pressure gauge that erroneously reads high pressure. The A&P checks this every year and notes that the oil pressure is within the normal range.
EDIT: "I guess I should be clear about what "erroneously high" means, the gauge pegs the needle so it isn't just a little high". – Ron Beyer
The sender has been replaced and the only thing left is the gauge. We've been flying like this for years. Recently one of our CFI's has determined that we are in violation of 91.205 and has told us to ground the aircraft until it can be repaired.
Have we been flying against FAR's? Is a "pressure system" meant to mean a pressurized oil system, or does it mean an aircraft with an altitude engine? Like I said, our A&P (two different ones in fact) have signed off on this since the engine was replaced 10-ish years ago.