W123 Oil pressure Gauge

[hms09]

While changing my instrument cluster on my 84 300DT, I noticed what looked like a small trace of oil on the bottom leather part of the cluster. Having read other posts, I think I have read that there is actually oil in the line that runs to the gauge. Is that true? Hard to believe there is actually oil in that line to read pressure. My other cars has a sensor under the hood that sends an electronic signal to the gauge.If this is true, I guess i have dislodged the oil line while replacing the lighting. ANy ideas/comments greatly appreciated/

Thanks,
Harry