Interesting thread..
Just a couple of thoughts,
The DIY oil industry is a trillion dollar market, and most of the advertising claims reflect the grab for market share.
Ultra low viscosity oils (0w/20, 5w/30 etc) were developed for below freezing starts and increased mileage benefits..and not extended engine longevity.
Running these oils at startup in moderate/warm climates means your running oil the consistancy of machine oil at a time when clearances haven't stabilised and lubrication is minimal.
Your engines lubricating system, oil pump and bearing clearances were not designed to run such oils.
In this country SAE recomends 10w/30 and 10w/40, like most manufacturers do for 90's vehicles, for ambient temps in most areas. better to stick with that range in a quality oil.
Oil viscosity ratings, and tests are conducted on clean oil stocks in a lab environment, and don't reflect oil thats been running in your engine for the last 2 months or more.
Using Delvac or any diesel oil will cause oil leaks in any mid to high mileage engine, as they contain a very high level of detergents.
Synthetic oils become contaminated just the same as dino oils do, with combustion by-products, blow-by, fuel contamination and condensation. Using extended change intervals means running contaminated oil for longer periods.
Running expensive syn racing oils for OEM recomended change intervals can be very expensive, and probably has a very limited benefit.
Filters are as important as the oil your using, maybe even more so, if you choose the wrong one.
There's no substitute for oil film strength in a perf engine, so viscosity should match the upper level recomended in the SAE range for the appropriate ambient temperature.
When it comes to recomended viscosities, who are you going to trust..the manufacturer (who pays for warranty claims) the oil companies? or the guy at the store.