The reason the modern engines use thinner oil can be stated in one word: efficiency. Efficiency all the way down the line.
Tighter clearances require less oil; less oil requires lower operating oil pressure & less volume of oil; less oil pressure means less hp to drive the oil pump; less hp to drive the pump means more hp to the wheels & less fuel used doing it. Same deal with electric cooling fans; they require a larger alternator, but overall they consume less hp than a mechanical fan. More hp to the wheels plus saving fuel. It is all for efficiency.
As far as start-up lifter noise is concerned, I believe it is a two edged sword. If the lifters have bled down, thin oil should pump them up quicker; but the thin oil is also more likely to bleed down the lifters after shut down.
I use 10w/50 oil & often don't fire up my 455 for 2 weeks. It fires instantly & I NEVER have any start up lifter noise. Didn't have start up noise either when I used the same brand 15w/60 oil. This the great Penrite oil. No friction modifiers in it AND it has Zinc D/T phoshate in it.
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