Originally Posted by
RVDT
AS365C1 TRGB in Oz for the same reason - "tide mark" - until it finally gave up gracefully.
Sight glasses can give you information other than the level.
On two occasions a colour change from light to dark lead to internal problems in Hughes 500 main gearboxes.
Metal in the oil that was not picked up by the chip plugs.
On another occasion the sight glass of an AS350 looked like a golden snow globe with particles glittering in the oil..
Another non ferrous internal failure not picked up by chip plugs.