Micra K 10 – Temperature?

Hi everybody, nice that I found this page. Drive a Nissan Micra K 10 (1.2 Topic, year of construction 90) and have a small problem. For some time the temperature (display C/H) is only very sporadically in the middle area, last it remains mostly in the “C-area” and moves only about 1 mm upwards (even after a longer ride). In addition (arguably in winter) the heating hardly comes to temperature. The problem was already in the summer and is curiously late When I turned up oil (probably a little too much) I asked: What can this be because there is a connection between the “too much oil-turned after” and this temperature not coming on the way (sowl at the display as well as at the heating) Thank you in advance for answers! Greeting Redfoot