These are just some memorable readings I've come across, worthy of saving.
Here is the best summary of motor oil topics that I've ever seen, written by a corvetteforum member, LDB. He's an oil company engineer, and kindly gave his permission to reproduce his work here. So stop by Corvette Forum some time, look him up and give him a word of thanks.
Original post at Corvetteforum.com, May 18, 2005.Go to TOPI posted the gasoline facts note a few days ago, and several asked about motor oil. In case you didnt see that post, Im an engineer for a major oil company, familiar with fuel and lube requirements. Mobil is not my company.
Before getting to Mobil 1, we need to understand a few basics about conventional motor oils. They are about 85-90% base oil from the refining process, about 2-5% of a long laundry list of additives (such as anti friction, anti oxidation, anti wear, carbon dispersing, detergent, and so forth), and about 5-10% viscosity index (VI) improver. That, naturally begs the question, what is VI?
Everybody knows what viscosity is - thickness. Syrup is viscous, water is not. When you buy an oil, the second digit in its designation refers to viscosity at running temperature. So if its 5W-30, the 30 is the viscosity at running temperature, which is what matters once your engine is warmed up. VI is viscosity index, referring to how much the viscosity changes with temperature.
Oil tends to get less viscous when hot, so the problem is, it tends to be too thick when the engine is cold (especially in super-cold temperatures in winter in the north), but then thins out and may be too thin when hot. In the old days, oil was low VI, so it had this problem in a big way. Modern processes can improve the VI, so that when it is cold, it behaves as though it is a thinner oil. When these high VI oils were introduced, a second digit was added to designation. So a current designation such as 5W-30 means that it behaves as though it is 5 viscosity when cold (the W stands for winter, ie, cold), but once warm, behaves as though it is 30 viscosity. The wider the spread between first and second numbers, the higher the VI.
As a bit of a digression, the reason that people once bragged about Pennsylvania Oil is that for reasons best known to mother nature, crude oils from Pennsylvania have the highest naturally occurring VI of any oil that has ever been discovered in the world. Thus, in 1950, before modern refining processes, if you had Pennsylvania oil, you were getting a 5W-30 oil even though the designation at that time didnt say so. Oils from other sources would have been 30W-30 had they been designated that way at the time. But with modern refining processes, you now end up at the same place by either route. And oh, by the way, almost all the Pennsylvania oil is now gone, so very little lube (not even very much of the Quaker State or Pennzoil brands) actually comes from Pennsylvania crude any more anyway.
But back to the main story line, what is VI improver? Some portion of the higher VI of modern oils comes from better refining processes such as extraction and hydrotreating. But those arent enough to reach 5W-30 on most oils. VI improvers are special chemicals with the unusual behavior that they get thicker, not thinner, when they heat up. So if you start from a 1950s low VI oil that is 30W-30, better refining can get you to perhaps 20W-30, and VI improvers get you the rest of the way to 5W-30. But theres a catch (isnt there always?). VI improvers have finite life, and once they wear out, they lose their ability to thicken the oil. Once that happens, your oil is too thin when hot. So what limits the length of time between oil changes is not the oil getting dirty, it is the VI improvers breaking down. If you left a conventional oil in for 15,000 miles, the VI improvers are mostly gone, and the 5W-30 oil is acting as though it is 5W-15, and you are potentially damaging your engine with oil that is too thin.
Now comes Mobil 1. It is not made from normal oil. It is a synthetic polymer with high natural VI, so it does not need VI improvers, and there is no risk of viscosity breakdown. It also has some other minor advantages, such as zero evaporative loss, better thermal stability, and less need of certain types of additives. But in terms of lubrication itself, differences are not large. If you want to run 15,000 miles between oil changes, you must use synthetic or you are in deep trouble with VI improver breakdown. But what if you are happy with 7500 mile oil change intervals? What, if any other advantages does Mobil 1 give? Directionally, its a smidgen better on almost all fronts. If you are truly an intense driver, in the sense of racing, or some other activity where the engine is running flat out for extended periods, its probably worth it for better very high temperature thermal stability, and for slightly better fouling resistance. But for the rest of us, if we are hard core about economics, its probably not worth it in hard nosed economic dollars.
The reality check is long distance truckers. They run highly stressed engines for multiple hundred thousand mile intervals between overhauls, and if there was a clear advantage in performance or longevity, you can be sure they would use it. They do not. Now admittedly, a big truck diesel uses a lot more oil than a car, so a truckers cost to run synthetic would be much higher than yours. But nevertheless, his pain if there is a failure is also much higher than yours. So its not a stupid analogy.
My bottom line recommendation on oil? Like with gasoline, dont buy a cheapie brand. Youre flirting with disaster on additives. If you use conventional oil, dont go beyond 7500 miles due to VI improver breakdown, or said another way, if you dont change oil often, use a synthetic. But for most of us, choice between Mobil 1 and a good conventional oil is more of an emotional choice than a choice that can be defended with hard numbers. If you use Mobil 1, you pay a couple hundred dollars more over the life of the car, and you get some small, but unarguably positive directional benefits in performance.
Go to TOPLots of gasoline comments lately. Im an engineer for a major oil company, familiar with engine testing for performance, economy, and emissions. I think my companys gasoline is slightly better than other majors, but Im not going to use the forum for advertising. Instead, Ill make a few points about what you should look for in a gasoline. If you do that, my company will get more than an average share of your business.
Octane is generally misunderstood. High octane gasoline isnt more powerful, it simply offers better resistance to engine knock. Lower than needed octane is a big penalty from knocking and/or spark retard, but higher than needed octane gives no benefits. Sounds odd, but its true as described below. Premium gasoline does have some benefits even to a car designed for regular, but those benefits arent from octane. They are from some other characteristics which are described at the very end, DI and additives. Lets talk octane first.
Two main factors set octane requirement: compression ratio and spark advance. Compression ratio is a design feature of the engine, with higher ratios giving better performance, but also requiring higher octane rating. Corvettes need premium gasoline because they are high compression ratio, which requires high octane. Best performance in any given engine is at a particular spark advance, and going in either direction makes poorer performance. But while it costs performance, less than optimum spark advance does have the advantage of lowering octane requirement. So your knock sensor can compensate for low octane fuel by retarding the spark, at a cost in performance. But the reverse is not true. Once octane is high enough to allow optimum spark advance, more octane doesnt help, because greater than optimum spark advance does not increase performance.
The Corvette gasoline spec is 91. There is some variability in engine response depending on a myriad of other characteristics including engine age, but its safe to say that benefits of 93 versus 91 are small, and 94 versus 93 are nil. You can make similar comments about the benefits of greater than 87 octane in a normal car. Its compression ratio is lower, so it can get to optimum spark advance on 87 octane. Running 93 octane will not help turn it into a Corvette, because it doesnt have the compression ratio to utilize the higher octane.
Bottom line on octane is that benefit of being over manufacturers recommendation is slim to none. The only three things that can cause you to get significant benefit from going higher are: 1) a 100,000 mile engine probably needs a couple of numbers more than new due to various irregularities in an old engine, 2) if you live at high altitude, a couple of numbers over normal spec are usually needed, or 3) if you do major engine modifications to raise compression ratio.
Other issues are a detergent additive package for valve and injector cleanliness, a reliable quality control system, and good DI, drivability index, a measure of how well the fuel evaporates. All majors are roughly equivalent in those areas, and most premium gasolines have more detergent and better DI than most regulars. There are shades of gray differences in additives, but once you are with a major oil company, you need careful tests to see them. All majors (including my own) have had quality control lapses, usually with sulfur (causes odor, emissions, and gas gauge problems), particulates (plugs filters prematurely), and/or water (the worst of all, and can devastate an engine, but this one is almost unheard of with the majors). But when the rare lapse does occur, we quickly correct the problem, including any damage it may have caused. When you buy from the grocery store, or Wal-Mart, or a convenience store, you are much more exposed. Some of the time, those places buy from the majors, and their gas is perfectly OK. But all too often, they buy on the spot market, the cheapest stuff that happens to be floating by on a barge. This gives you higher probability of a serious quality control incident, and also exposes you to poor additives and DI. In the short term, youd never notice lack of detergent additives, but over time, you engine will foul. DI symptoms are subtle, mostly a rough warm up period, but poor DI also tends to foul the engine, especially if it is short on detergent. So you have to ask yourself, do you want to fuel a $50,000 car from the lowest bidder.
Anyway, at the very end of this tome, Id advise you to set you minimum standard as being at least recommended octane, from one of the majors. Once you are at that point, differences become small. Does 93 versus 91 or 94 versus 93 make much difference? Probably not, unless you live in the mountains or have an aging engine. Is premium worth it in your regular car for the slightly higher additive concentration and slightly better DI if you dont need the octane? You have to make that decision. There are benefits, but you are into small stuff. Its kind of like Mobil 1 versus normal oil. Sure, Mobil 1 is very slightly better. Its also $4 versus $1 per quart. But thats another long post, and Im out of energy for now.