The main reason 5W-20 or 0W-20 oil was specified for your engine is to increase the CAFE (Corporate Average Fuel Economy) reported to the Federal Government.

CAFE is the combined average fuel economy of all of a vehicle manufacturers product line. Minimum CAFE levels are specified by the Federal Government.

In order for a vehicle manufacturer to continue selling profitable large trucks and SUV's, which typically have poor fuel mileage ratings, as compared to smaller cars, and still meet mandated CAFE requirements, they must also sell enough of the smaller cars which have much better fuel economy ratings to offset the poor fuel economy ratings of the larger vehicles.

For model year 2001, the change to a 5W-20 oil will allow Honda and Ford's overall CAFE to increase by a very small amount, typically in the tenths of a mile per gallon range

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That may be a factor but it also has to do with many of the newer vehicles relying on oil pressure and specific viscosity to do certain things, like the timing chain tensioners in the 4.0 SOHC V6's or the VCT solenoids and variable cam timing phasers in some of the V8's. Also the tighter tolerances used in the manufacturing of todays engines require the use of such oil weights.

 

However, for an engine designed in the mid/late 80's, 3.0l V6, even with the marginal update tweaks, it's really not going to be that happy with water when it's hot.

Sorry for necro, but i figured this was a good place to make the point.

 

Actually.... Thinner oil flows better, more oil = more cooling. Modern oils have VERY tough film strength, even for thinner weights. Running 5-20, even 0-20 in your Vulcan V6 is perfectly OK provided it is mechanically sound and not already having bearing/cam/lifter issues.

 

For 2001 V6~4.0L~SOHC I've got here with Oil+Filter changes at 6mth\5kMiles:
1st ~10yrs ran 5w30.Blended & spec Filter MotorCraft#fl820s per owners manual,
2nd ~10yrs run 0w30.FullSyn Summer & 0w20.FullSyn Winter & K&N#hp2011 Filter.
Now using thinner 0w30\0w20 oil for 2 reasons: living in a 9kFt altitude cooler climate,
& having become paranoid about getting oil to TimingTensioners ASAP during cold starts.
If I lived in a typical warmer climate, I'd use 0w30.FullSyn year round.
Prefer the K&N#hp2011 filter over the spec MotorCraft or others, because it has a
EasyOff hex nut & slightly higher flow rating & slightly lower bypass pressure rating.

Taking a wider view, it's likely most important just to do frequent oil+filter changes,
irregardless of what oil weight or filter used.

More info at
https://2001fordexplorersporttrac.fandom.com/Engine_Oil