Oil temperature

Can anyone help?
I have an ex army 90 which has the discovery 200tdi engine fitted, I am hoping to use the oil temp gauge but once I had purchased and fitted the oil temp sensor the gauge slowly rises to above the red over temp section of the gauge although the oil is not hot.
Is it likely to be a faulty gauge? could I swap the water temp gauge with the oil temp gauge to assist in troubleshooting?
surely the oil cant be that much hotter than in the 2.5 n/a /2.5td than the 200tdi engine oil tempereature? any help would be much appreciated

Could just be that the sender and gauge are not calibrated the same

Quite common with water temp gauges when swapped between engines