I agree that modern engines have better tolerance control then older engines, and work just as well with lighter viscosity oils. One additional item to ponder, lighter weight oils also have less drag and can provide a measurable improvement in fuel economy. Here in the deep south USA, we tend to use the heavier viscosity oils, as allowed in many of the owners manuals. My auto manual states that it requires 5W20, I routinely use 10W30, but I also like to use semi-synthetic at 5,000 mile intervals.
Finally, as to "Enginebobs" comment about late model cars with oil controls, I frequently visit the dyno labs at a VERY modern engine research facility, and also review the emission results. I almost never see a smoker there, and if I do, the emission results are way out of range. Oil control is not such a problem as it has been in the past. The "smokers" tend to be from high mileage autos which have had less then ideal service, abused, poorly maintained, etc. A recent comment overheard at a manufacturers conference relayed this: "Our engine warranty's due to oil consumption is almost nill". A quick look at vehicles which have failed roadside emission tests reveal my previous comments to be true, and also, a very high number of them with "Check Engine Lights" illuminated.
The auto of today is without a doubt better built than those of 15 years ago, or even 10 years ago.
Franz
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