Someone explain my delta temps

[bosax]

A couple of months ago, I experienced my first 15deg delta temp. Since then they have slowly crept up. I start figuring out it's time to change out the oil cooler (first time, 114k). So I change the coolant to ELC and install a coolant filter. I figure on running this for a thousand miles before switching out the oil and EGR cooler.
Today I was taking a trip to the beach (100 miles) and had for the first time a 20deg delta. However, the ECT never got above 184, and 184-204 was the difference most of the way. Coming back with temps a humid 85, I did the the ECT to 190, but did not seem to affect the EOT.
I would say I need a new thermostat to go with everything. But will this actually change the deltas? I don't want to spend thousands of dollars replacing crap if all I need is a new thermostat. And I have never had the EOT above 215deg.

[PGreenSVT]

Change the thermostat and retest.

[WarChild]

correct! ECT should always be 190 or higher that is the normal operating temp.