The newer cars use lighter weight oils for one reason only.......... ...mileage. You save maybe $7 a year in fuel cost, at least what I figured once at around 15K miles a year. In my view the other reason is because the oils wear the engines slightly faster-they are trying to make up for cars lasting too long now. They did their job too well for once, the PCM keeps them running forever now. Take note of how much talk now about 'startup rattle' like it's been there forever but it hasn't, only since the oils got so light. I have actually cured that rattle by going to higher vis oil and forget the $7. You never really heard of anti-drainback valves in filters until the thinner oils showed up, even though several engines used them WAY back. Now everything pretty much does.
The conventional wisdom says thicker oil is harder to pump up when cold. True.........B UTTT..........
Thinner oil can run out of the bearing clearances easier when capillary attraction quits easier, a function of all fluids in relation to viscosity. Funny how they never tell you that part in the ads..........i f the oil is thicker it hasn't backdrained out of the bearings...... ........
The friction modifiers they replaced the zinc with work pretty well on older stuff unless you start running killer valvetrain loads like on oldschool cam rubbing surfaces. You can find more expensive motorcycle specialty oils that still have zinc but you pay a premium for it now. Truck oils like Delo and Rotella have pretty much lost their extra zinc as well now too.
Certain types of motorcycle have one way clutch type starting systems that if you use a modern car oil with no zinc can then begin to slip to not start, the friction modifiers work that well. TOO well. Changing to correct oil then puts the starter back right again.
Myself, I still use 10-30 or 10-40 oils and will go no lighter, or my old stand by straight 30 weight. Got it in like 3 cars right now and can't kill them. I'd go straight 40 here in Texas in the summer if it wasn't so expensive now. Conventional oil, not synthetic, I haven't gone there yet. At around 9000 mile oil change intervals now I make money even with the oil changes. Can't put my fingers on it exactly but some of the mods they've made to oil in the last few years have also transferred to the conventional oils as well in my view, they won't tell you that of course because they want you to justify all that investment by buying synthetic. I'm beginning to think some of that friction-modifier-zinc-replacement technology IS synthetic and they have to put it in everything but don't say. The oils just don't burn black nearly as quickly as they used to and I'm talking still in the realm of recent PCM controlled cars, not the old carbed ones. At the 9000 number I quoted above the oil will still be brown not nearly black and be still about 80% transparent with very little opacity. Why I started stretching the oil changes further and further, an experiment. I had been running 6000 for many years. Pulled pan on one @ 150K, NO sludge in bottom, I'm talking NONE, light film only that rinsed right off. WALMART Supertech straight 30 weight! Crap oil!
Not saying you all should go and do this but there's food for thought in there somewhere..... .......the fed regulations that oil makers dump zinc and increase mileage may have brought on much better product overall, even the cheap crap.