When should you start using 20w 50 oil in a 1997 lancer with 180000km will it do any good to the car?

It isn't a matter of doing it any good, it's more of a method to get more miles out of a worn engine. If you've been regularly changing the oil, the bearings should be good and seldom experience significant wear. Often improper maintenance or extremely high miles will result in worn bearings and a significant reduction in oil pressure. Once the oil pressure starts to signficantly drop, it's time to change to a higher viscosity engine oil.

Usually, the signs will be a very low oil pressure on the gauge or the oil pressure light starts flickering or even turns on solid when the engine is idling and the oil is warm. That is a sign that the clearances between the bearing surfaces and the crankshaft have become too great and that the oil is too thin to maintain pressure. Slightly higher viscosity oil solves that problem, at least for a while longer.

It's not a good idea to change to a higher viscosity oil before it is necessary. The thicker oil is harder to move and slightly reduces fuel economy, and can take a little longer to fill the oil journals on a tight (not worn) engine, potentially causing premature wear.

That's an excellent question, and it's good to keep an eye on the condition of your vehicle.