Interesting Oil Info

[Hellbender]

Some kinda interesting stuff about oil......

"Ever since making their debut in aviation some 20 years ago, multiviscosity oils have steadily gained in popularity to the point where now, it is almost axiomatic among pilots (if not mechanics) that the best oils are multigrades. Tending to reinforce this viewpoint is the fact that multivis oils are generally more expensive than their straight-weight counterparts. (It's natural to assume that if something is more expensive, it must somehow be better.)

On the other hand, it may well be that straight-weight oils provide better protection against wear than multigrades, a hypothesis that many in the oil industry seem reluctant to consider, but that may have to be reconsidered in the light of a new study published by the prestigious Society of Automotive Engineers.

SAE Findings


Last March, a pair of Mercedes-Benz Engineers (Rudolf Thom and Karl Kollman)along with pair of Shell Oil engineers (Wolfgang Warnecke and Mike Frend)wrote an SAE technical paper (No. 951035) on "Extended Oil Drain Intervals: Conservation of Resources, or Reduction of Engine Life?" Among the many interesting results presented in the paper was a graph showing cylinder-wall wear-rate Measurements correlated with cylinder wall temperature. Cylinder wear rates(in micrograms per hour) were monitored as a function of cylinder-wall temperature in a test engine specifically fitted with radionuclide-impregnated cylinder liners. Three test were done so that three different kinds of oils could be used. In one test sequence, a straight 30 weight oil was used; in another, 10W-30 multigrade; and in the third, straight 10 weight oil. (The test engine was a Mercedes-Benz OM 616 2.4-liter, 4cylinder diesel.) In each case, the engine was operated at fixed speed, torque and temperature conditions until constant wear rates were observed. Wear rates were then plotted against cylinder wall temperature.

The graph of cylinder-wall wear rate versus cylinder wall temperature tends to be bathtub-shaped, with wear increasing sharply at each temperature extreme (as you'd expect). But while two of the oils turned in very similar wear performance, one oil stood out as protecting the engine against wear at the extremes of temperature. That oil was plain SAE 30 (Straight-grade 30-weight). At either extreme of temperature, the maximum wear rate with 10W-30 was more than double that of the straight SAE30 oil. (The worst performance was turned in by straight 10-weight.)

These findings should come as no surprise, since in general, thicker oils make for thicker oil films, and the thicker the oil film, the better the wear protection. What's surprising is that a 10W-30 oil (which is supposed to have viscosity comparable to an SAE 30 oil at high temperatures) does not provide at least equal wear protection to that of a 30 weight unmodified oil. The simplest explanation, it appears, is that the base stock from which a 10W-30 multigrade is made, namely, 10 weight oil, is not as good a lubricant as 30-weight base stock. Also it would appear that viscosity-index (VI) improvers are not, in themselves, robust lubricants.

It is important to understand how a multiviscosity oil is made. A 10W-30 multivis oil (for example) actually starts life as a 10-weight oil. Prior to fortifying the oil with additives, it consist of a straight SAE 10 base stock. Viscosity-index improvers and pour-point depressants (waxy long chain polymers) are added to give the oil added thickness at high temperature, When the 10-weight base stock would otherwise thin out terribly. The reason VI improvers work is that they change shape, depending on temperature. At low temperatures, they collapse into little balls of yarn (or at least, that's how you can picture them); at higher temperatures, they unfold into long, spindly, spaghetti-like molecules that behave a lot like normal oil molecules. At lower temps, in the collapsed state, the ball-like forms act like little ball bearings, easily sliding past each other. This is what allows the combination of a thin base stock (SAE 5, 10, or 20) and VI improvers to act like a thin oil at low temperature, but thicker oil at higher temperatures.

The trouble with VI improvers is that they are not shear-stable; they break easily once unfolded into the long, spindly, spaghetti-like form. They also combust easily, to form soot and other byproducts. And according to SAE 951035, they don't lubricate as well as an SAE oil of similar grade.

Conclusion

Our recommendation on oils is as follows:

Use a straight-weight ashless dispersant oil when outside air temperatures are above freezing. Preheat the engine whenever OAT's are below 25?F.

In the winter months, or as OAT's consistently fall below the freezing mark, use 40 weight oil, or (if OAT's are consistently below 10?F) switch to a multigrade (for example, Phillips X/C or Shell 15W-50. But continue to preheat as necessary. Switch back to single-weight oil in the summer. "

Kinda makes sense to me,
HB

[Jays89YJ]

Interesting. Too bad no engine runs at a constant rpm and torque like that. I think advanced testing is definitely needed. Perhaps starting, driving conditions, temperature variations, shutting down, etc. This experiment is done in an ideal environment with ideal variables.

Cool read though!

[Hellbender]

Seems everyone is saying that "90% of engine wear is during start-up".

It would be interesting to see some tests done that compare how fast (after engine shut-down) the lubrication layer drains off wear surfaces comparing 5-30 or 10-30 and straight 30 wt. oils. 1 hour, 4 hours, over night, etc, etc.

I used to put oil-pressure switches on high-dollar engines that would not allow start-up until 15 psi of oil pressure was reached by cranking.

HB

[Jays89YJ]

Originally posted by Hellbender
I used to put oil-pressure switches on high-dollar engines that would not allow start-up until 15 psi of oil pressure was reached by cranking.

HB

Now that is something I should look into! Especially since my Jeep sits for so long.