A couple of articles on synthetic v's non-synthetic oils
From Car Craft magazine:
Over the years, Car Craft has tested many different types of engine components. A common theme underlying many of these tests is that bigger is not necessarily better, especially on the street. But just as many continue to believe in rad cams and giant carbs, traditional, thick, single-viscosity oils still have a strong following among car-crafting traditionalists.
Of course, high-viscosity oils don't flow well at low temperatures. In the old days, guys living in cold climates put in a thinner oil for the winter with a "W" or cold temperature-viscosity rating. Although they poured better at low temperatures, straight-viscosity "W" oils, in turn, didn't do a good job of protecting high-performance engines once they reached normal operating temperatures, so they weren't recommended for sustained high-speed driving. The oil industry developed "all-season" multiviscosity oils to solve the problem, but some of the early products didn't hold up under heavy-duty operating conditions, tainting the reputation of multiviscosity lubricants among many Car Crafters to this day.
Yet today's modern oils are vastly improved over those of 20 years ago. For oils that meet the current "SJ" service designation, viscosity breakdown is no longer a significant problem, thanks to modern formulation technologies and viscosity enhancers. Auto manufacturers have also redesigned their engines for tighter clearances and instituted precision machining techniques that take advantage of thinner oil to deliver improved fuel economy through reduced friction.
Like the OEMs, racers have discovered that friction reductions plus precision tight clearances yield greater efficiency and more power. Racers also know that most engine wear occurs at start-up, so it's critical that engine parts receive proper lubrication as soon as possible--hence the need for an initially thinner, so-called "winter" viscosity. Today, few racers run a single-viscosity motor oil except nitro-burners. According to 76 Lubricants, most NASCAR teams use the really thin stuff during qualifying, moving up to 20W-50 during the long race (although it's rumored some teams may use the extreme cold-weather thin oils all the time, but don't want to admit to their latest performance "trick").
Synthetic oils, pioneered in the '70s by Mobil and now available from most major oil companies, take the all-season, multiviscosity approach to the outer limits. Unlike traditional mineral oils that are produced by distillation and further refining of existing crude oil stock, synthetic lubricants are made through chemical reactions. These new oils aren't synthetic or artificial in the sense that they're manufactured out of whole cloth--they still have the same natural ingredients found in "real" oil. But in a synthetic lubricant, these ingredients are recombined like a Lego set to yield synthesized-hydrocarbon molecular chains with desirable characteristics and uniformity not found in even the highest-quality traditional motor oils. Typically, the best synthetic oils use a combination of up to three different synthetic base fluids--polyalphaolefin (PAO), synthetic esters, and alkylated aromatics.
Because a synthetic oil's molecules are much more consistent in size and shape, they are better able to withstand extreme engine temperatures. By contrast, the unstable molecules in conventional oil can easily vaporize or oxidize in extreme heat. Mobil 1 synthetic is said to be capable of protecting engines "at well over 400 degrees F"; in the real world, most racers have no problem running synthetics up to 290 degrees F under prolonged use, but they get really jumpy when a conventional exceeds 270 degrees F.
Because a synthetic oil is chemically produced, there are no contaminants in the oil. By contrast, conventional oils contain small amounts of sulfur, wax, and asphaltic material that can promote detonation as well as varnish and sludge buildup. With no wax, synthetics will flow at much lower temperatures than conventional oils. In fact, synthetic oils are now available with viscosity ratings as low as 0W-30, as in Mobil 1's new Tri-Synthetic blend or Castrol Formula SLX. These oils flow more than seven times faster than a conventional 5W-30 motor oil during initial start-up, yet at normal operating temperatures act like a regular Grade 30 oil.
An 0W-30 synthetic oil is capable of pumping easily at -62 degrees F and flowing at even lower temperatures. Conventional oils are essentially frozen solid at that temperature, so there's simply no conventional equivalent to this new grade. There are 5W-30 conventional and synthetic oils, but even here, the synthetic has a real-world advantage: Mobil 1's 5W-30 will pump at -58-degrees F, compared to about -35-degrees F for a conventional oil.
But claims and talk are cheap, so Car Craft had Westech Performance run some of the new Mobil 1 0W-30 in Ford's prototype 392 small-block stroker crate engine. The Mobil 1 was compared to the generic (and recommended for this engine) 20W-50 factory-fill conventional oil, as well as 10W-30 conventional oil. All tests began with the oil temperature stabilized at 210 degrees F. The engine ran from 3,300-6,200 rpm, and several runs were made for each oil to ensure repeatability.
In terms of peak numbers, we found that the engine gained nearly 7 hp with the thinner conventional oil, and was up nearly 10 hp with the synthetic. No peak torque gains were observed by changing from 20W-50 to 10W-30 conventional; however, the synthetic was up 15 lb-ft of torque at the peak. Looking at average numbers helps explain where the gains occurred--both the thinner conventional and synthetic oils broadened the torque and power bands overall, but the thin Mobil 1 showed the greatest improvement under 4,700 rpm, indicating that the thinner oil provides less initial drag for the engine to overcome.
However, thinner oil also translates to lower oil pressure: The 0W-30 oil developed 10 psi less than the baseline 20W-50. Only 46 psi was on tap at 6,200 rpm--kind of shaky as most gearheads like to see at least 10 psi per 1,000 rpm. Still, the engine ran OK, and the bearings looked fine on teardown, seemingly verifying synthetic manufacturers' claims that their products' greater shear strength more than makes up for lower viscosity. Is 10 hp and 15 lb-ft worth paying two to four times more for a quart of oil? Or the potential for extended engine life? You be the judge.
From Popular Mechanics magazine:
Q: After a recent trip to two major auto parts dealers for oil (5W 30), I realized synthetic oil is slowly replacing nonsynthetic on the display shelves. The problem is, I'm getting conflicting information about synthetic oil. One store attendant told me I shouldn't go back to regular oil after changing to synthetic. Then he said it would be okay to add a quart of regular oil to synthetic, if I needed to top up. Another clerk said I should never mix the two. At a different store, the employee said it didn't matter if I used synthetic and then later replaced it with regular oil.
A: Early synthetics got a bad reputation for leaking. This was because, despite the claims of the oil manufacturers, the seal-swell characteristics of the new synthetics were different from those of the mineral oils they replaced. If the seal-swell rate was lower, the seals shrank and oil leaked from crankshaft seals and rocker cover seals. If the rate was higher, the seals swelled a little extra and the engine was tight. Then if the owner changed back to mineral oil, or added a quart when no synthetic was to be had, things got really bad. The crank seals had become worn, in their turgid state, and then relaxed. The valve cover seals were compressed when swelled, and when the different oil was added, everything leaked like, well, an old English sports car.
Fortunately, the situation has improved; you should have no problem switching back and forth. Adding a quart of mineral oil to a crankcase full of synthetic will be fine. Read the fine print -- a lot of the "synthetics" on the market are blends containing a substantial proportion of mineral oil.
cost me a new clutch that did!
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