The manual calls for 5w30, can I use 5w20?

Use 30.

If you have an oil temperature gauge, yes; if not, iffy unless you know that you will not be pushing the engine in terms of how hot it might get.

Use what the manual dictates.

It depends on temperature, vehicle loads, operating speed.

Surrounding temperature
- Lower temperature: use lighter oil
- Higher temperature: heavier oil

For towing/increased loads
-Higher load: heaver oil
-Light load: lighter oil

Operating speed
-Higher operating speed: lighter oil
-Lower operating speed: heavier oil

+1

Why try to re-engineer?

Go ask this over at www.bobistheoilguy.com Very knowledgable folks on oil over there.

Usually, the manual recommends the thinnest oil that will provide adequate lubrication under all anticipated driving conditions. This helps maximize fuel economy, and helps the manufacturer meet CAFE requirements. Going to a slightly thicker weight oil may reduce fuel economy a tad, but it's rarely harmful to the engine.

However, going to a slightly thinner weight oil could result in inadequate lubrication under certain conditions.

BOTTOM LINE: Not a good idea.

Defiantly wouldn't do it, they make the manuals for a reason, follow it.  I have a 04' ranger with the 4.0, it gets the 5w30.  Perfect oil for all around use, My cousin has a 94' ranger, and that's what he uses, all be it the high mileage version, the thing has damn near 200,000 and no major engine problems, good trucks!

+1

If you've got a calibrate oil pressure and temp gauge (the oem dash gauge is NOT calibrated enough) you can do your experimenting and MAYBE NOT harm your engine.

Thinner (lower weight) oil flows better.  Thicker Oil (higher weight) protects better.

Usually the owner's manual has a recommendation based off the temperature, use what is recommended by the manufacturer for the temperatures you drive in.  If you have a graph or range that you end up falling into that two different weights of oil will work, choose the thicker oil for better protection.

You only need thinner oil for cold start-up, to make sure it flows.

Yes, thinner oil can give you better fuel economy, but we are talking a fraction of MPG here, at the cost of better protecting your motor.  As well, the more worn an engine, the worse the mileage it gets.  So you can save 1/5 mpg now, and then lose 5 mpg 100k miles from now.  OR you can lose that 1/5 mpg now, and only lose 1 mpg 100k miles from now.  Take your choice, I'd recommend choosing the weight of oil that will protect the motor the best, NOT give you an extra fraction of a mpg at the cost protecting the motor less.

Remember the manufacturer has CAFE to meet, been given all sorts of incentives to encourage you to save energy and they would prefer you buy a new car in a few years, they are biased toward recommending the lighter less protecting oil that gives a fraction of MPG.

10W-30 oil means, 10 means @ 0°F the oil has the same viscosity as standard straight "10" weight oil, W means it has passed a winter flow test of flowing out of the can at -20°F, 30 means @ 200°F the oil has the same viscosity as standard straight "30" weight oil.

NO, the oil does NOT get thicker (more viscous) as it gets hotter, 30 weight oil is thinner at 200° than 10 weight oil is at 0°F, it means the oil thins out at a slower rate as it gets hotter, than straight weight oil.

The oil achieves that with a bunch of additives, and the additive that does the multi-viscosity are polymer chains that coil up cold and expand when hot, these additive are the first to burn and coke up the oil and make sludge.

SO, if you can do it, the closer the bottom and top range of the oil, the less additive in the oil, thus less additive to burn and create deposits and get the motor dirty. i.e. 10W30 is better than 5W30, because of the narrower spread.

Use what the manual says, for the temperatures you drive the vehicle, if you fall into a range where you can choose, choose the higher viscosity and then the viscosity with the narrowest range, if the temps you drive at support it.

No, you cannot use 5W20.  See this TSB:

http://www.ints.com/support/documents/Ford5W-20motoroil.pdf

The 2001 Ranger 4.0l is listed as 5W30 only.

TYCOM

I use Valvoline 20/50. I always have.

So, if you have 5w30 (as recommended by your manual for your usage) in your engine and you are out in the boondocks, oil level below the add mark, and all you have is a quart of 5w20, then yes, I would put in the 5w20.  Better the wrong oil or not enough oil.

But if you can get the right oil, do so.

I use 5w20 in my 4 cyl ranger (factory spec'd) and its like freakin water.  There must be some tight tolerances in that motor, because there's no way I'd put that in an engine not designed for it.