Few realize that hard, amber varnish on engine parts is not just dirt but oxidized oil and fuel residues that can seed fresh deposits. In Nordic stop‑start use, short runs and moisture make this film stubborn, undermining clearances and heat transfer. A solvent pre‑cleaner can loosen varnish and tar so the next oil change removes more of it. Use it with care and drain promptly. This approach can be paired with Tar remover and engine cleaner.