The mechanical systems were accurate. They received the pressure through a tube from the oil pump. But sadly, like the one-shot oiler, the systems are inherently super risky as the plastic tubing is vulnerable to heat and other causes of breakage. When this happens, they cause pressurized oil to spray about and empty the engine of motor oil (with dire consequences) in no time..putting the engine (and yourself) at dire risk from an engine seizure. Without oil, engine parts expand and lock. When traveling, it is a pain to address this issue and clean up its mess.
However, there are fittings that can "splice" the tube together with the appropriate pressure fitting. The systems come from the home oil furnace industry and are still used theough they have long since been outlawed for cars. So they can be had if you find a specialist in home oil furnaces. They are little brass fittings consisting of two pressure dowels (inside), a male threaded fitting and two fmeal threaded fittings. To repair, one must make clean cut ends, then take the fitting apart, slide the male fittings on each side of the tube, then place a dowel on a side, push a bit of the tube into the male threaded fitting and thread the female bit on over the dowel and onto the male fitting. Now do the same on the other side and you have your splice. If there is insufficient length, then buy a bit of an extension from the same source you got the fitting and buy two fittings, one for each end of the splice. If securely attached to the car or motor, it makes a permanent repair...albeit with the same risks as the original.
varying
resistance. This eliminates the tube filled with pressurized oil so 
that
risks and consequence of that type of breakage is eliminated. Replacements
gauge can be had, indistinguishable from the originals, that function electrically
rather than mechanically. However, there are some issues that must be checked
and confirmed. The sender, the oil pump and the gauge must match or be
made to match so the readings make sense, the range of readings span the
specific motor pressures, and the threads in the oil pump and sender fit
(which can be done with adapters if necessary. These senders have all sorts
of shapes..most often like a mushroom. Some will have two posts for
wires (a ground/earth feed and a wire to the gauge) or one post (only a
wire to the gauge as it assumbed the sender is grounded/earthed by the
contact of the threads with the oil pump).