The oil acts as a damper to limit needle oscillation. It performs a function similar to the shock absorbers sometimes used on vehicles....without them, your car or truck wants to continue bouncing when you drive over a bump or pothole. Some types of air compressors, especially piston types, do not provide a continuous supply of air, rather they exhaust (pressure) on one stroke, and intake (vacuum or negative pressure), on the other stroke. This makes pulses in the air supply, and the gauge will register these pulses with rapid needle movement.

A liquid filled gauge slows down the action of the needle, so it remains at a more constant reading. greatly increasing gauge life, as well as making it a whole lot easier to read.

Liquid filled gauges usually have a space or bubble at the top, to allow for expansion and contraction of the liquid. I've never had to fill one, but most I've seen have around 1/8 to 1/4" space.

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.