Depending on which year you are talking about. I have a '98 Cobra and the block is an aluminum Teksid Block. The new supercharged blocks are cast iron. The pistons are domed forged units.

 

The Terminator engines (Supercharged) should have dished pistons for lower compression which are forged along with a cast iron block and a steel crank.
The 5.4L is taller and it takes longer for the oil to reach the heads
Also, the Cobras are made to be driven in nice weather, not in the winter time muck. (at least mine would be)
The hand build Cobra engines probably have different bearing clearances too.

 

I looked at engine bay pics from the online versions of several magazines, as well as any specs I could find, and all that information I saw on 4.6 Cobra engines actually had "5W-XX"...

I was trying to justify the comment made by ford_man_of_vass about 15W in a Cobra engine, but what I found only made me think that a 5W base was the factory approved oil for Cobras. Why they would ship them other wise has me confused?

Anyway, I was going with the aluminum block and forged pistons over a cast iron block and hypereutectic pistons as the reason for the 15W, but since Ford doesn't build the Cobra engines with aluminum blocks anymore, that kinda throws that theory.
One of the reasons for the 5W-20 or 30 was the Tritons use hypereutectic pistons, and with a very tight tolerance... you kinda need the thinner oil.

For those that don't know, I pulled this definition from some on-line Wikipedia type joint:
Hypereutectic pistons are castpistons made from aluminum with over 16% silicon content for strength and durability. The term 'hypereutectic' comes from eutectic.
Special melting processes are necessary to ‘super-saturate’ the aluminum with additional silicon content. Special molds, casting and cooling techniques are required to obtain finely and uniformly dispersed silicon particles throughout the material.
These newer pistons are very hard, thus brittle. They have proven to be un-forgiving with engine knocking. For this reason they are great in naturally-aspirated engines, but should never be used with 'nitrous', super- or turbochargers. Generally speaking, forged pistons are a better choice for high boost.
Hypereutectic pistons are used in some original equipment engines. They are favored because of reduced scuffing, improved power, fuel economy and emissions. The reduced thermal expansion rate allows the piston to be run with reduced clearance.

The reduced emissions is the key reason Ford uses these pistons. The Hypereutectic pistons and the reduced clearences that they allow is the reason Ford uses 5W-20 today. The 5W-30 is perfectly acceptable if the truck calls for it. I'm not sure if tolerences in these engines became a slight bit "tighter" as the availability of 5W-20 became greater. If forged pistons were used, then 15W would be an understandable, although odd, alternative. Using the 15W in these engines would not be an engine life extending act.

-Kerry

 

[QUOTE=kspilkinton]I looked at engine bay pics from the online versions of several magazines, as well as any specs I could find, and all that information I saw on 4.6 Cobra engines actually had "5W-XX"...

I was trying to justify the comment made by ford_man_of_vass about 15W in a Cobra engine, but what I found only made me think that a 5W base was the factory approved oil for Cobras. Why they would ship them other wise has

I would have 2 agree. No such mention of 15W in my Cobra

 

ford fills the cobra with 5w20. you can read the ford publication.

 

im running 5w20 amsoil in my 4.6 04 heritage 24000 miles on my 4 oilchange and is comming out prety clean omost looks like new oil is draining out the oil pan.

 

2003 4.6L 5w-20 Mobil 1 full syn 9,000 miles, great engine.
1986 4.9L new long block 10W-30 Mobil 1 full syn 200 miles, ran great today,ran 180 miles.
1991 460 cu 10W-30 Mobile 1 full syn, 68,000 miles, great engine.
1998 Kia 4cyl 1.9L DOHC 10W-30 Mobil 1 full syn, 67,000 miles,great engine.

 

M1 10W-30 in my blown 5.4L

 

past the second oil change...

MC 5/20 and OE Motorcraft Filter...

 

I don't think it will hurt at all. Where i work we have 5 superdutys from 99 to 2008 all of them have 5.4 in them. They run 15w40 in them as soon as the warranty was out. 4 of the have over 100,000 miles do not take a drop between oil changes no cold engine knock motors are way quieter then mine which next oil change i will be running 15w40. The 2008 is still under warranty but as soon as its done 15w40 will be put in that. They run 15w40 in all the crown vic cop cars also. This weight oil is run year around and i live in pa where it does get cold here from time to time. Just my 2 cents.

 

5w-30 Valvoline Durablend with 1 quart Lucas oil treatment in my 99 4.6 F-150. Almost at 200,000

 

2003 Supercrew FX4 - 5.4L- Amsoil 5W-30. Have 106,000 miles on the truck and still running strong.

 

pennzoil 5w20 with motorcraft 820s filter.

 

2000 f350 5.4 110k....Factory sticker under hood recommends motorcraft 5w30 but finding it in gallon jugs is impossible. All local parts and dept stores carry gallon jugs of 5w20 but only quarts of 5w30 both synth. Buying by the quart is significantly more expensive.

 

Motorcraft 5w-20 with the Motorcraft filter to keep it clean...