An oleo-pneumatic shock absorber having a casing, a rod slidably mounted in the casing, first and second fluids in the casing, and a gauge. The gauge has a substrate positioned on the shock absorber. The substrate has a first region with a slot sized to fit an ultrasonic transducer arranged to encompass a range of possible oil levels within the shock absorber, a second region corresponding to a range of possible extension states of the shock absorber and a third region comprising one or more traces. Each trace corresponds to a temperature value and is indicative of an optimum relationship between the oil levels and the range of extension states at the respective temperature value associated with the trace.