10W30 is what this motor was originally designed to run on. Then somewhere along the line, after the motor had been in service in the minivan fleet for some time, Chrysler reduced the specified weight to 5W30, and then reduced it again to 5W20 when this motor went into the then new JK Wrangler..... Chrysler did not make any major changes in design specs or internal tolerances to account for the change to this drastically thinner oil which is why these motors consume oil through the PCV system and past the rings and suffer carbon fouling in the combustion chambers, excessive pinging, blocked cats & failed O2 sensors, and many other issues associated with oil burning. Chrysler reduced the oil weight specification for the Wrangler-fitted 3.8L V6 to 5W20 purely for mileage reasons and nothing else (when these motors are given the 36hr bench test they produce slightly better mileage figures running the thinner oil and thus help Chrysler to meet it's federally-mandated CAFE mileage targets for the fleet), and know very well that it results in high oil consumption which is why they claim oil consumption of one quart per 1000 miles is "acceptable".

You get about 1% better fuel economy, but you get 30% shorter engine life !

SAE 5W-20 or even SAE 0W-20 are becoming more popular these days and were specified by some OEM's (FORD & HONDA) on new cars since 2001 Model Year.
Although these oils are promoted as "energy conserving" they generally trade a gain of less than 0.1 MPG in Corporate Average Fuel Economy (CAFE) for shorter useful engine life.
FORD which has previously designed cars to have 10 year or 150,000 miles life has reduced the mileage life expectation to "beyond 100,000 miles" on vehicles that are operated on SAE 5W-20 Motor Oil.
HONDA only claims "useful life" as 7-years or 70,000 miles in EPA certifications for their CIVIC which uses SAE 5W-20 Motor Oil, while the previous model year that utilized SAE 5W-30 Motor Oil was certified for 10 year or 100,000 mile durability.
Since both HONDA and FORD Warranty their NEW cars for ONLY 3-years or 36,000-miles the reduction in engine life expectancy is not a factor.
By contrast Mercedes-Benz recommends use of ONLY Synthetic Motor Oil that is at least SAE 5W-40 !
This is a recent increase in recommended viscosity from SAE 5W-30. Apparently customer research indicated that engine longevity is more important to typical MB customer than fuel economy.