10W-30 is probably okay for where you are located. If you drive north however I would go with a thinner oil if you do so during the winter.

By the way the lower viscosity oils will create less drag in the engine making for more power.

stalag

Either 5w20 or 5w30 should be ok. 20w40 is a no go.

why and how?

By the way the lower viscosity oils will create less drag in the engine making for more power. It is a fluid dynamics thing.

stalag

at the same time if they viscosity is very low for an extended period of time its a bad thing

for example : 5w20 + 90 degree heeat + 5 hour drive might not be a good thing for obvious reasons

20W oil can be worked in an engine up to 120F ambient. Unless you are doing extended long distance driving like a truck your assumption is not true with most conditions encountered in the US. The sole exception may be Death Valley in the dead of summer which would require 5w-30.

Stalag

Ok I keep 5w30 in my 98 marquis year around now. I used to put 10w30 in it year around, then I found out 5w30 is good in between all purpose oil for this kind of car.Heavy oils are for older cars and vehicles under lots of workload stress. So suit yourself.

Part of Ford's 5W-20 testing involved running Expeditions and Navigators, loaded to maximum capacity, for 250,000 miles with 15,000 mile OCIs. If it can pass something as tough as that, and has Ford's backing, then I'm sure Death Valley wouldn't be an exception.

Even at this end, where temperatures get up to 60 C, I have yet to hear of an oil related failure with 5W-20 in Fords.

IDK, I'm using 5W-20 in my 04 Grand Marquis. I regularly drive from So. California to Arizona. It's about 6 or 7 hour drive (depending on traffic) and last summer it was 110F. and above for most of the trip. (I saw as high as 115)
That was half a dozen trips ago.

+1

i just think there must be a downside.. im not saying for sure..

i personally run 5w-30

Landyacht,

Actually there is no downside. The tolerances are so much tighter in a 4.6L modular engine (5.4L as well) as opposed to older designs that the thinner oils are more than up to the task of protecting the engine. The tighter tolerances means that the oil films need to be correspondingly thinner as well. The film surface is what allows shear to occur between two moving parts. That shear occurs easier in a lighter weight oil reducing internal friction of the fluid and as a result gives just as good lubrication when the tolerances are tighter. The thicker oils are used where shock protection from parts that are moving to keep them from scuffing/banging together when you have loose tolerances. Also, a high quality synthetic will beat out a conventional oil any day of the week because the designed properties of a mixture of known chemicals verses a weighted fraction from oil distillation allow for better shear and surface protection. Lighter oils are also less prone to production of sludge (because of the better flow properties of the lighter oils leading to less contact time with hot surfaces) and as a result are also better for the longevity of a vehicle.

To make the arguements you are attempting you need an understanding of fluid dynamics and film surfaces to be right on this point. I did, when I was in high school 20 years ago, play with different oils in a 1.6L Subaru Brat. At that time the vehicle called for a 10W-40 weight oil. I found the following points.

1. Heavier oils like 20W-50 did work but bogged the engine.
2. 10W-30 worked reasonably well and was better than 10W-40 as far as performance.
3. Mobil 1 5W-20 found all the leaks in the engine but also cleaned the crankcase and the engine ran much cooler. After a couple of oil changes at 3000K intervals the oil came out much cleaner. I also had a cousin who helped me do the valve lash adjustment on the car comment on how clean the valve assembly was in comparison to other engines he had worked on. Also, I saw the best performance with that oil in my crankcase when I drove the vehicle.

Today being a scientist (PhD) crosstrained as an engineer I now understand why this all works. I would suggest courses in fluid dynamics and study of surface structures before making a vague statement like

" I just think there must be a downside. "

Stalag

thank you i was basically hoping someone would "school me" on this.

i dont understand fuild dynamics at all, but i do understand the hotter the oil gets the more it thins out...which made me think that it could have negative results.

i made a vague statement because i only vaguely understand it..
if we all were scientists and mechanical engineers there would be no point of this site.

thank you though that does make a little sense to me , do you have any links to some studies or anything like that ? im interested to know more about this, as i was arguing with my teacher about it today, and he has managed to convince the whole class that running heavy oil in the summer / long trips is healthyer for the motor ..

There are entire textbooks at the college level dedicated to fluid dynamics. Much of the math if you are in high school is even well above your teacher and your teacher may have some trouble comprehending. PM me if you want to chat a bit about this topic.

Stalag

Found this to be an interesting read

http://ferrarichat.com/forum/faq.php?faq=haas_articles#faq_motor_oil_basics