Hi Holy cow,
This all depends on the manufacturer of the engine. If 0W20 is allowed, this means that with this oil in all circumstances there will be enough pressure left.
My Civic can take 0W20 and at next oil change I will put it in.
Pay attention to what you call a 'worn engine'. In my understanding that's an engine where the clearance between pistons and cylinders is to big. If in your understanding a worn engine is one where the clearances in the bearings are bigger, that's another story. You should find out where pressure is lost.

I don't see how worn cylinders will affect oil pressure. The blowby might affect crankcase pressure, and more oil might be burned, but the pistons do not receive directly-fed oil as far as I know...

-BC

Oh no Bob, I didn't mean you will loose pressure when pistons/cylinders are worn.
I meant that you will consume some oil with thinner oil when pistons/cylinders are worn, because the bottom oil piston ring will not be able to do his work anymore.

That's right... Thicker oil is more difficult to pump through small openings so you end up with more pressure than the same engine running a thinner oil. As bearings open up pressure with the same viscosity will fall.

The difference between 0 and 10 will be almost nothing as far as engine protection is concerned although you may notice a performance increase. You're talking about when the engine is cold and even 0 weight is thicker in a cold engine than 50 weight in a hot engine. I noticed it in the Tracker but am pretty sure I wouldn't in the Durango as I don't even notice the AC cycling in that thing.

Well, Jones, I picked the '0WX' because with lower cold resistance the oil will always be pumped quicker around the entire engine than a '10WX'-oil. That's where the difference in protection should be, especially with this car that's always driven cold. In these circumstances there's a risk that the (OH-)camshaft receives very late or insufficient lubrication time after time.
Her previous car started consuming some oil after approx. 50Kmiles, same bad circumstances. Now that the warranty period is over I can put in it what I want: only the best for our conditions, within the manufacturers margin. I don't want this car to see consuming oil so quick.

A good anti-drainback valve should prevent the oil from draining out of the passages. Since you have an OHC engine I can see it being more of an issue.

The viscosity of oil is the HIGHER NUMBER so a 0W-30 has the same viscosity of a 10W-30. It is not thinner. The "W" stands for "Winter" to signify how it flows at lower temps.

20W good to 32 degrees F
15W good to 14 degrees F
5W good to 0 degrees F
0W good below 0 degrees F

(degrees from memory, but you get the idea)

Thickness is approximately the same as viscosity in these discussions.

As for the ratings...It's multi-viscosity. The number before the W is its viscosity rating in low temperatures. The number after the W is its viscosity rating once it's warmed up.

I could go on, but here's a link where somebody wrote it accurately and succinctly:
https://www.repairfaq.org/filipg/AUTO/F_oil_facts.html

Both numbers are a measure of viscosity, or at least a standard labeling system corresponding to viscosity. Manufacturers should post actual measurements for viscosity under product specs. THC's Castrol falls in the 30 range, but is at the high end, close to 40.

Apparently, 'cold' viscosity testing is done at 40C. For the Celsius challenged, that's 104F. Publish results for Mobil1 10w30, 5w30, and 0w30 are really close, but I wonder how much they'll diverge under real world cold temps.

I've heard that 0w isn't possible with dino oil. So, if you want a true group IV synthetic, or least higher percentage of it, grab the 0w.

Yes Darby, thanks, but nobody ever doubted this? I can only confirm.