5W-30 was original factory recommended oil - then ford went to 5W-20 to save fuel. So either is ok. Use synthetic or sythetic blend if you can afford it (mobil 1 or castrol, etc)
You should use what your car recommends 5w-20 is thinner than 5w-30.
5w-20 5w-20
"To protect your engine's warranty use Motorcraft SAE 5W-20 or an equivalent 5W-20 oil meeting Ford specification WSS-M2C930-A. SAE 5W-20 oil provides optimum fuel economy and durability performance meeting all requirements for your vehicle's engine".
Personally , I would use the 5W-30 as stated in the owners manual , it's not until the 2001 Ford Ranger model that it shows to use 5W-20 in the 3.0 liter V6 engine
According to the 2002 Mercury Sable Owner Guide ( 5W-20 ) With engine oil filter change : For the 3.0 litre " Vulcan " V6 , and its flex fuel version ( 4.5 U.S. quarts of 5W-20 engine oil ) For the 3.0 litre " Duratec " V6 ( 5.5 U.S. quarts of 5W-20 engine oil )
use what is specified for your engine - by the manufacturer
SAE 5W-30
5w-30 or 5w-20
5w-30 or 5w-20
5w-20 or 5w-30 depending on climate
Use exactly what the manufacture recommends.