Thanks Captain.
You're right the best location for the oil temp sensor is the bottom of the sump. If you have a horizontal drain plug, drilling out the plug for the sensor works well.
Having said that, your oil temp readings sound about right. My temps in my Caterham and Bimmer show a similar pattern.
About 180F on the street and that can take 1/2 hour of highway speeds to attain and in the winter it's very hard to get any temperature into the oil.
If the people on this website knew that in all likelihood the oil temps in normally aspirated engines frequently does not get to the optimum 210F I think there would be less resistance to 20 wt oil.
At 180F the operational kinematic viscosity of the PP 0W-20 is 12.6 cSt using the Widman chart or effectively a light 40 wt oil and at 140F it's 21.9 cSt, a 60 wt oil. So, in effect the PP 0W-20 is too heavy an oil in your Sentra and very much so in the winter.
The RP 0W-10 was actually a good choice being effective a 30 wt (9.7 cSt) oil at 180F but still too heavy at 140F effectively a 50 wt (17.5 cSt) oil.