After a blown head gasket.. or just after years you need to clean the system.. MB has cleaners and others are on the market.. You need to get the oil film out of the cooling system.. If you coat the inside of the cooling system with a film of oil.. It will not exchange heat as well.. Thats fact.. Often a system that just heats some and every thing else is right just needs to have the oils that have gotten inside cleaned out.. Just changing coolent will not do the job.. and they always need to be cleaned after replacing head gaskets, radiatior, ect.. All the engines of the last 15 or more years have computers of some type.. If you run them cooler than they call for they will not perform well as the computer does not take over until they are up to the given temp.. Below that they stay on a start up program.. So its best to stay with the Thermostat Called for on your car..