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Originally Posted by aelfwyne
The idea behind this basically is that thinner oils offer less resistance inside the engine. Typically non-synthetic oils are only available down to 5w20 viscocity rating, whereas synthetic is available in a 0w?? rating (not sure what the higher number is). This is because supposedly, synthetics will lubricate better with less viscocity, which allows lower viscocity (thickness). Therefore, there will be less resistance in the engine from the oil, and it will run more efficiently.
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The smaller number (before the w) is the cold viscosity, and the larger number (after the w) is the hot viscosity. Cold viscosity is what matters when you first start your engine (because thick cold oil can harm an engine when first starting, so the smaller the "cold" number the better), and after several minutes (when the engine is fully "warmed up") the large hot viscosity number is what matters (because at that point you are at "steady state" and want an oil thick enough to do the needed work, but not so thick you are wasting effort/FE dealing with that thickness).
Yes, one advantage of synthetics is that because they are generally better lubricants, you can (in theory) use a lower viscosity (i.e. thinner oil), and still have OK protection. However, even at the same viscosity, a synthetic will generally have lower friction (and that can effect FE some). Also, synthetics generally clean an engine better, and a cleaner engine should (in theory) run better (and get better FE).
So I think you are better off with synthetics FE wise, even at the same viscosity. The effect isn't huge (I think Mobil advertises an average 3% gain for their Mobil-1 oil vs dino oil), but many people (myself included) think the effect is real. And if you use the fact that synthetic oil is higher quality to also lower the viscosity (i.e. use thinner oil), you can increase FE even more in many cases.
BTW: If you pay attention to the additive packs (which often means buying the same basic type and brand of oil), there is nothing wrong with mixing oil weights, to get the thickness you are looking for (especially when the weight you are looking for isn't sold in the stores). For example, I've recently started adding a little (fully synthetic) 5w20 oil to the mix, to thin out my 5w30 (which is the majority of my oil) just a little bit (i.e. giving myself essentially a 5w28, or thereabouts)...
In addition, synthetic oils generally cool better than dino oils. And if the oil is cooling better, more of the engine (or transmission, if you have synthetic transmission fluid) heat will be handled by the oil (and less will have to be handled by the "cooling system"). While this is "a good thing" overall, it probably doesn't make a huge FE difference, but it could easily make a small one. For example, if more of the engine is being cooled by the oil, the radiator fan might have to run less often (which means a little less electrical usage, which means a little less alternator drag on the engine).
BTW: It's been a while since I switched over to synthetics, but I remember the increase in "power" being quite noticeable. And my old gas logs seem to indicate that the car's average FE went up a little after the switch over as well (however I didn't run a lot of tankfuls on the old "dino oil", so there is always the normal fluctuation between tankfuls to consider). Still, a good synthetic does seem to help in my personal experience.
NOTE: Be aware that "high mileage" cars will typically leak a LOT when first switching over to synthetic. But after the additives in most good synthetics work to refurbish the seals, this extra leaking often goes away on its own. Also, because synthetic usually clean better, long standing sludge/varnish can brake loose as well (quickly clogging oil filters, or worse, when you first switch over). For both of those reasons, use short (2k-3k) oil change intervals for the first couple of oil changes, when first switching over a "well used vehicle" to synthetics. But the "good news" is that after you get over this initial "cut over period" the vehicles will generally be happier on the synthetics.
NOTE: Once your engine is cleaned up, you can often run the synthetics much longer than dino oil (10k+ per oil change is common, and 20k+ isn't unheard of with top quality oil and filters). And this much longer time between oil changes can easily save you more money than the cost difference (vs using cheap oil/filters, and changing it more often). But to take advantage of this, you have to make sure you have enough oil (check the dip-stick, if you have any leaks), and you have to use quality oil filters (to keep the crud out of the oil). Because if/when the oil gets dirty, it's time to do an oil change (even if the oil was otherwise OK). So by using extra good filtration, dirt is no longer the reason why you have to change your oil (and other factors, such as when the oil's additive pack is used up, become the key reason to need an oil change).
FWIW: The current theory over on "Bob is the oil guy" forums, is that the new Amsoil EAO (they use a new nano-fiber synthetic media) oil filters (not Amsoil's older filter model, the new EAO filters only), are about the best "full flow" filters you can easily find on the market. They aren't cheap (about $10/each at the "wholesale price" available to "preferred customers" of Amsoil, and about $15/each retail). However, the specs on them are VERY impressive (Amsoil rates them for 1 year or 25k miles, they have a good flow rate, AND impressively fine filtration). I've just started using them, and so far the car is very happy (but I haven't had them on very long yet). Still others over on BITOG have used them longer then I have (and even cut them open, to examine the results better), and many BITOG members are impressed with what these filters can do!
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