"Some think that engine wear during the first 20 minutes after start up is due to oil falling off the engine parts when at rest. I have never seen this in any scientific studies or in any SAE papers. I do not think this is relevant at all."
'Good' multigrades seem to reduce wear, but in aviation the results sem mixed:
http://www.ingentaconnect.com/content/mcb/018/2000/00000052/00000006/art00004
Modern heavy duty diesel engine cold start wear study
Abstract: Increasingly stringent European market requirements for improved heavy duty diesel engine performance, such as fuel economy and durability, continue to highlight the need for higher performance engine lubricants. To meet this need, new additive technology is being used in combination with higher quality base-stocks and has resulted in a growing trend towards the use of oils of lower viscosities than those used in the past. Such a trend has led to some concern over the ability of low viscous oils to maintain adequate engine wear protection, not only during extended service operations but also under the more severe conditions of low temperature engine start-up. This paper describes the results of a recent study to evaluate the effect of base oil composition and viscosity grade on the low temperature performance of a modern Euro II diesel engine with respect to wear, using the technique of Thin Layer Activation. Engine test results showed that the use of full synthetic SAE 5W40 grade oils based on Polyalphaolefin provided enhanced low temperature cylinder liner wear protection.
http://www.eaa49.av.org/techart/str_oil.htm
Do straight-weight oils provide better engine protection?
Our own experience (based on incoming calls to our engine hotline) tends to confirm that lubrication-related problemsespecially involving the camshaftare generally much less among users of straight-weight oils than among users of multigrade oils. There is nothing scientific about our sampling method, admittedly. But we do routinely ask callers what kind of oil they are using, and how often they fly. The people with serious engine problems (scuffed cam lobes, spalled lifters, etc.) we've noticed, are virtually always using a multigrade oil and/ flying less than 100 hours per year. (The number of people calling our hot line who scuff a cam while using straight-weight oil is negligible.) There could be many reasond for the apparent better performance of single-weight oils. It may be for instance, that operators who use straight-weight oils are more careful about preheating in cold weather (or tend not to live in cold areas at all) and therefore have less exposure to cold-start damage. Or it may be that operators who fly very little are more likely than most to buy "fancy" oils, in hopes that this will better protect their engines against the inevitable consequences of inactivity. (We have noticed that the multigrades do seem to have achived a very high market share among inactive or barely-active owner-operators.)
On the other hand, it may well be that straight-weight oils provide better protection against wear than multigradesa 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, 10w30 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 10w30 was more than double that of the straight SAE30 oil. (The worst performance was turned in by straight 10-weight.)
......Until now, we've always believed that to the extent multigrades might not be protecting camshafts as well as straight-weight oils, it might simply be because multigrades which are thinner at ambient temperatures than most straight-weight oils simply run off of the parts quicker in an inactive engine, leaving steel parts exposed to the elements. However, SAE 951035 suggests that even in an active, running engine, straight-weight oils protect against wear better than multigrades. (Remember, in the SAE study, actual wear rates were measured while the engine was running.)
