While it is true that the extreme pressure lubrication properties of many engine oils began to change in the mid-1990s and changed significantly in the early 00s, the recent story in Corvette Fever magazine did as much to propagate myths and dispense incorrect information about how current oil blends affect valvetrain wear in engines with overhead valves and flat tappet cams as it did to dispel some of the rumor and urban legend about those subjects.
In the above referenced article, Corvette Fever said that synthetic engine oil is "too slippery" and that prevents lifter rotation.
It is, in my opinion, inexcusable for a magazine which fashions itself as a technical reference for the Corvette enthusiast community to publish such a distortion of fact. That said, I’m sure the magazine didn’t do that intentionally. It is possible this happened due to poor research and inadequate fact-checking, problems which are far too common in the car magazine business these days.
Some in media must think that, during the 90 or so years pushrod valvetrains have been used in automotive engines, engine manufacturers have never done any research. They choose believe the wild-assed-guess some knucklehead engine rebuilder makes when confronted with a camshaft failure—that the root cause was the synthetic oil in the engine being too slippery?! Oh please...spare me.
To some people who understand engine oil, the chemical and mechanical processes at work inside a running engine and, specifically, lubrication at the lobe/lifter interface in a flat tappet valvetrain; the idea that any oil could be "too slippery" and such "excessive slipperiness" interferes with the mechanical action which causes lifters to rotate is laughable...it’s pure nonsense.
A 1992 SAE paper GM issued offers information about oil film thicknesses and frictional properties. It said that there are two cam angles where there is zero oil film, but most cam angles have some oil film. This paper also indicates that, if the friction of the follower in the bore is greater than the cam lifter friction, then slip will occur. While, in theory, that slip could occur, in practice, as long as the engine has no mechanical problems, the friction at the lobe/lifter interface will always be greater—much greater, actually—than friction between the lifter bore and the lifter. Remember: the same oil is lubricating the cam/lifter contact and the lifter/bore contact.
A second paper, published by Ford, indicates that lifter/bore friction is about 13% of total at low speeds and goes down to about 2% at higher speeds. Those numbers in mind, in theory, you would need a reduction in friction of nearly 80% with synthetic oil for slip to occur. With synthetic oil compared to petroleum-based oil, you can see reductions in valvetrain friction of, at most, 10%. So, in theory, it ain’t gonna happen. Oh, but forget the theories. In practice, even if there was the necessary 80% difference in friction between synthetic and petro-based oil, for that difference to result in slip at the lobe/lifter interface, you’d need to have the lobe lubricated with synthetic and the lifter lubricated with conventional oil which is, of course, impossible.
With respect to lubrication at the lobe/lifter interface when the valvetrain is highly loaded, it is not a film of oil which prevents metal-to-metal contact. It is a near-monomolecular layer of iron sulfide and iron phosphate which provides the extreme pressure lubrication necessary to prevent rapid wear.
Iron sulfide and iron phosphate form when the sulfur and phosphorous resulting from the breakdown of zincdithiophosphate (ZDP), blended into the oil as an extreme pressure additive, combine with the iron of the cam and lifters. At the lobe/lifter interface, temperature and pressure cause ZDP to break-down into alcohol (which evaporates), zinc (most of which washes away), sulfur and phosphorous. Those last two chemicals combine with the iron molecules on the surface of the lobe and and the face of the lifter to make an extremely thin layer of iron sulfide and iron phosphate and those two compounds provide the necessary lubrication.
Most of the oil is actually "squeezed" out or wiped away and under some conditions, there is virtually no oil film at all. The sulfur and phosphorous provide lubrication in a process which "trades" chemical wear for adhesive wear, thus preventing failure of the lobe and lifter.
Since, in a practical sense, there's no film of oil available to be "too slippery", there you have further evidence that an engine which suffers problems with lifter rotation can not have this "slipperiness" as the root cause of the problem.
Ok...so what can cause reduced or lack of lifter rotation?
It could be a camshaft with lobes already worn due to other problems such that they have inadequate taper left to cause lifter rotation. It could be a lifter already worn such that it's convex surface (which also facilitates the rotation) has become "flat". It could be that either the cam or the lifter, or both had manufacturing defects such proper lifter rotation didn't occur. It could be that the cam and/or lifters experienced rapid wear due to improper installation.
It could be a block with worn or damaged lifter bores. It could be liters with worn or damaged "barrels". It could even be a block that was machined incorrectly such that the lifter centerlines are not offset far enough from the centers of the lobe surfaces for adequate liter rotation to occur. Admittedly, that last scenario is a bit far-fetched but it could, in theory, occur.
But...if a flat tappet engine has lifters that don't rotate, it's not because an engine oil was "too slippery".
I hate to say this because, in years past, Corvette Fever has been a customer of mine, but...in my opinion, its publishers need to take steps which could prevent such myths being published "as fact" in technical articles. If the magazine is incapable of that, then I think that, perhaps, it should do the Corvette hobby a favor and confine its content to car features and event coverage.
