water level stays high

Volvo VNL series
Cumminx ISX 400
750,000 miles

Few months back I started having exaust leak. about 2 months back I got it fixed. but my mechanic did some mistake. I drove 600 miles and I checked next morning during PTI that my coolant in the coolant reservoire was all black. I drove to the nearby dealership they found Oil mixed in the coolant. Dealer fixed it and charge for 17 hrs. Now here is my truck problem starts. 1) Normally my water level used to stayed 170 for light to medium load. now it stays 185 all the time.
2) When I start my truck all gauges cycle through but the Water level.

 

I got it checked by professional mechanic and it is found that Thermostate was bad.