another one for the experts...

[rebreath]

Today I dismantled an oil gauge from a 1977 something.not sure if it was a 240d or some kind of 300d,got the gauge from a forum member(jim16671836) and was told that I could swap the gauge mechanism for the one in my dash(which stayed fixed until it got cold?!).here is my question...the gauge face reads differently than the one in my dash,after the switch my gauge reads at about the #2 mark.before the switch it read closer to the #1 mark.could this difference be because of the SLIGHT leak that had developed or is it because the tubes are calibrated differently(to coincide with different markings on the 77 gauge face).where should it sit at idle when hot? thanks for any and all input...