Have a look here for basic info on oil
http://www.pqiamerica.com/apiserviceclass.htm
have a read of this link , towards the end their some links on oil pressure vs temp .
http://www.ft86club.com/forums/showthread.php?t=81493
very rough giude
If your car is fairly stock and you just drive on the street in a cold to mild climate then just stick with the 0W20 synthetic as per manufactures recommendations for car.
If you drive the car hard or track car in warm to hot climates (ambient well over 30C and never gets below 0C) then probably a 5W30 or even 10W30 is better suited especially if you not running any form of oil cooler.
If you run an oil cooler and your keeping your oil temps below 115 C at all times then you can probably go back to 0W20, if your seeing greater than 120 C then either get a bigger/better oil cooler or run a slightly thicker oil like 5W30.
Its always better to keep the oil temps under control than to run really thick oils
modern synthetic oils dont break down till over 150C, its not generally the oil breaking down you need to worry about its the oil pressure dropping due oil thinning at high temps.
You rearly need to monitor your oil temps to determine the most suitable oil for your conditions and driving