Everyone is in agreement that too much heat is bad for the engine and transmission. Another fact most people are in agreement with is every degree above 200 is a worse and worse situation. 180-200 seems to be that sweet spot for the engine. For the Ecoboost engine in particular 190-200 seems to be the best temperature. However the 2nd generation Ecoboost in particular comes with these active grill shutters designed to keep the engine hot. These are great for cold weather climates like Canada but horrible if its not cold outside. The only purpose of the shutters is for the governments CAFE meaning Ford trys to squeeze out every last bit of MPG they can not caring about reliability. Therefore if you not in a cold climate I suggest disconnecting the shutters. Its fairly easy. The only hard part is getting out the plastic retainers and I suggest using a specific tool so you dont damage them. My suggestion is to disconnect the shutters then monitor temperature. If it stays between 190-200 you are fine. However if it pops over 200 then move to either a 180 or 170 degree thermostat. Most owners will do well with the 180 and Motorcraft makes one that will fit. If you order the thermostat for the 2019 Lincoln MKT Livery they will send you a 180 degree which will fit. However those who 1) do a lot of towing or 2) modify or tune the engine or 3) find themselves in a climate over 90 degrees like Arizona or 4) race or track their vehicle will want the 170 degree thermostat. Casual and commuting truck owners will do well with a 180 degree thermostat whereas more aggressive owners who push their truck to extremes will need the 170. I have the 170 degree thermostat and have disconnected the.shutters but the engine coolant is constantly in the 180s and I suspect it would be ideal if it were to hang in the 190s. Even pushing the vehicle hard in Excite (Sport) mode in 90 degree weather it gets to 198! Never made it over 200! So I think I would be better off with 180 degree thermostat.