Lyle is a very respected LS mechanic, and brings up some very good points about oil viscosity. Skip to 4:33 for the oil piece. Hell, I don't care if you put olive oil in your own car, but I always prefer to buy cars that have been maintained to spec, and with proof.

 

same LS engines in Europe C6 spec 0w-40, 5w-30 in the U.S. is for fleet CAFE mileage goals by GM. 5w-30 is fine but if you drive your LS hard 0w-40 or 5w-40 is preferable.

 

I use what the book suggests! Probably change more often than the computer says!! Kind of wierd though that my Corvette says and comes with Mobile1 on the LS3 but the Camaro i had (same engine) does not say Mobile1!!!

 

Just thought I'd share my experiences... I am a OEM product design engineer and in the 90s I had about 5yrs of engine design experience (including bearing specs). I'm not an oil expert nor am I a bearing expert. But I know the design aspects of displacing loads from a rotating shaft into a stationary / rigid support. In those efforts I worked with bearing suppliers who were the experts. I'd give them the design criteria and they would then advised me on required journal sizes, roller / journal types and sizes, quality of lubricant, seal design, material type, and calculated life expectancy.
My summary / opinion / experience for this discussion of 5W-40
First understand what a bearing does: There are four most basic criteria of a bearing.
1) Loading needs to be transferred from things like rotating shafts, skids like pushrod to rocker, pistons / bores like lifters and combustion pistons, ect..... Always remember those little parts that get lubricated too!
2) Shaft speeds. The speed of a shaft has an effect on friction. The higher the heat the more metal expands which causes bearing clearances to change. It's NOT always a reduction in clearances!
3) Expected life. Life cycle is extremely complicated due to all the variables like cold starts (-30C to full hot restarts) with ONE OIL viscosity. New engine vs 200k mile worn engines. Consumers who use basic oils (non-synthetic) don't change their oil for 15k miles, never check the fluid level, known loading in all temps and speeds, Amount of particulates in the oil (especially synthetic oil).
All contact bearings wear. Even journals that float on a cushion of oil. The largest wear takes place a startup after a long shutdown. The other condition that wears bearings is a lack of lubricity (excessive loading from preignition, initial nitrous shot at low rpms, excessive engine RPM, ect..), Oil / grease that has too many particulates in it (excessively dirty), oil that has low viscosity for the load / temp,
4) Temperatures. Metals expand and contract w/heat. Oil is more viscous when its cold and less when its hot. Mutli viscosity oils have additives that help but break down quickly w/excessive heat. Synthetics are far superior with heat.

I've not changed my 04Z to 5W-40 yet but I will soon and here is why...
#1 I change my oil often. And in all my cars. Even the daily driver family car I don't go more than 5k miles on a full synthetic oil. My two toy cars get changed about every 3k miles as I don't drive them very often and I don't want the oil to get acidic (A V8 Jag and my Z06)
#2 I've experimented a lot over the yrs w/differing oils and viscosities. Initial start piston slap, valvetrain clatter, and both at full operating temp are things I specifically isten for.
#3 I'm value minded. I've not considered 0W/5W-40 oils in the past due to a 50% cost premium. But now... prices are pretty close. Last time I looked my local box store had all M1 shown at the exact same price for all viscosities.
#4 over the last yr I've changed the Jag and my DD to 5W-40. And I must say I am impressed. The engine turns over just as easy as running a 5W-20/30. My DD has had a valve train rattle and piston slap during warm up and is even a little noticeable after full heat. It's now 90% gone at full heat and 100% gone at cold ambient temps (and I've tried 10W40 in the past). The Jag always had a lifter tap from about 10sec after start up to about 2mins after start. It's now 100% gone and is incredibly quiet all the time. We just drove it 2200 miles on an out of state trip and its been perfect the whole time (engine is scec'd for 5W-30 and in it I've tried differing weights and even custom blended my own)

In the C5Z I have 46k miles and the last two yrs I've experimented w/differing weights. As it sits right now I have about 2,800 miles on a 60% load of 10W-40 and 40% of 10W-30. It's much quieter at startup and significantly quieter at full heat. (I think the aluminum block makes the LS1/6 sound louder than a iron block).
This fall I'll service it and give 5W-40 M1 a try. (it never sees below 30F starts)

Just my 2 cents....

 

Your professional experience mimics my empiric experience.

 

same LS engines in Europe spec 0w/40. In the states they are trying to get the CAFE fleet mileage up so the engineers probably lost that battle.