170 litres!! What activity is going to heat that amount of oil and how long will it take to do so non stop

Put some alloy fins on the tank or use an inline finned type of passive cooler.
Adding additional electrical sensors, controls and wiring just adds unnecessary complexity which is the problem with so many items of heavy equipment nowadays.
Keep it simple, if it needs any additional cooling at all, a passive solution should be enough. Stick it in the radiator fan airflow if you really think you'll get enough heat. Oh and you can always paint some shiny tish on it