I don't know... I agree a lot of "engineering" goes into oil viscosity for engines. I agree 10W-30 would give you better "fuel economy" compared to 15w-40. However, what's good for one thing isn't always good for another. If you put 0W-30 in your axle it'll give you better fuel economy... but see what will happen to the gears... that's why we run 80-90. I think that's an extreme anology of the viscosity in a motor. Car companies are trying to meet CAFE. company average fuel economy and they're in the business of SELLING CARS; don't forget that. So I tend to take what the car companies say with a grain of salt. if I bought a brand new car tomorrow that said 5-20... Fine... I'll run 5-20 in it probly for 100K then go to 5-30 at that point. Another thing to remember... a motor with 150K has more wear than a new motor. so If they call for 5w-20 and 150K later you still think you need to run 5-20... you're agreeing if you took the motor apart it'll have the same tolerances and specs as it did when new, which is BS. so as the car gets older it gets more wear and loses lubrication protection. And... Pressure is resistance to flow... so if you run 15-40 in it @ 150K... it'll make up some of the wear restoring factory PSI/lubrication. I do agree 20W-50 is very thick for "today's" engines. Another thought... if 10W-30 was "the perfect viscosity oil" why don't the diesels that drive thousands of miles everyday recomend 0-30? they drive for a living, think how much money they'd save on fuel costs! If they could save some fuel by running a thinner oil I'm sure they'd probably do it. Even a 2006 Cummins Dodge, duramax chevy and ford powerstroke call for 15w-40. So to be honest with you... I think 10-30 is the best "compromise" oil weight. the engine engineers see that it's enough protection so the motor won't burn up (and wear is "probly calculated") and fuel is being saved. Look at European cars... they're made with tight tolearances... and they call for 0-40 or 5-40. and gas is $8/gallon over there... so i'm sure saving fuel would be on their priorty list. But one thing seperates europe from america... They keep a car 2-3x's longer than the average american. And they drive HARD. I was just over there this summer visting family. My cousin is a Engineer over there and he runs thick ass oil in his turbo Pegot. So if we had to motors off the factory floor and filled one with 5-20 and the other with 15w-40 and drove them hard as hell through the mountains of europe at sustained speeds over 120mph daily I'll bet my money on the 15w-40 motor. Why? thicker oil has more protection... can't aruge that. As long as it's not "to thick". Why don't we run 0-20 in our bearings to save on fuel??? b/c the oil isn't strong enough. But some RWD's use 80-90 for the axle and it also lubes the rear wheel bearings. As for changing oil every 3K or 3 months I agree that is BS. In europe most cars recomend 1yr/10K miles (I converted KM to miles). Dino or Syn. I think the best thing to do is LOOK at your oil to see if it needs changed. I've seen so many differnt shades of BLK oil, I could probly tell you when the oil needs changed by looking at it. Granted, the BEST way of knowing when is oil analysis. But for the average joe... why not look at the oil carefully. if at 4K it looks clean... why change it? My brothers Trans Am has that GM oil change light system and it'll come on about every 4,500. and the oil looks clean as a whistle. my 1984 supra FSM says to look at the oil to see if it's in need of changing! How many manuals say that?! the 1984 Supra FSM recomended Visc chart says for temps over 80F 10W-50 or 20W-50. I wish I could put this chart up here for people to see. I never looked at it til now.
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Seems to work okay for the time being