Oil Facts v1.1
filed in 1994 Ford F150
Question: To help fill the never-ending search for knowledge which is USENET:-}, the following info sheet (FAQ if you wish) is being posted to rec.motorcycles and rec.autos.tech monthly.
More Than You Ever Wanted to Know About Motor Oil by Ed Hackett
Choosing the best motor oil is a topic that comes up frequently in discussions between motoheads, whether they are talking about motorcycles or cars. The following article is intended to help you make a choice based on more than the advertising hype.
Oil companies provide data on their oils most often referred to as “typical inspection data”. This is an average of the actual physical and a few common chemical properties of their oils. This information is available to the public through their distributors or by writing or calling the company directly. I have compiled a list of the most popular, premium oils so that a ready comparison can be made. If your favorite oil is not on the list get the data from the distributor and use what I have as a data base.
This article is going to look at six of the most important properties of a motor oil readily available to the public: viscosity, viscosity index (VI), flash point, pour point, % sulfated ash, and % zinc.
Viscosity is a measure of the “flowability” of an oil. More specifically, it is the property of an oil to develop and maintain a certain amount of shearing stress dependent on flow, and then to offer continued resistance to flow. Thicker oils generally have a higher viscosity, and thinner oils a lower viscosity. This is the most important property for an engine. An oil with too low a viscosity can shear and loose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm.
The weights given on oils are arbitrary numbers assigned by the S.A.E. (Society of Automotive Engineers). These numbers correspond to “real” viscosity, as measured by several accepted techniques. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50 by the S.A.E. The W means the oil meets specifications for viscosity at 0 F and is therefore suitable for Winter use.
The following chart shows the relationship of “real” viscosity to their S.A.E. assigned numbers. The relationship of gear oils to engine oils is also shown.
First of all, thanks for the tremendously informative posting!
I live in sunny Orlando, FL, where the temp rarely goes below freezing. Although I an happy with my choice of Mobil 1 (high VI and ZERO ash), I am still at a loss of which viscosity to choose.
Now the questions:
1) Since Mobil 1 uses no polymer viscosity improvers, can I assume that a large viscosity range is not a disadvantage (e.g.5w30)?
2) Is it true that if the oil can hold up under temp, that the thinnest possible oil is desired?
3) Do you agree with the following post: Mobil 1 products, in my opinion, have very good thermal properties,but very poor shear properties…and yes, I have conducted some low level experimentation on several oils at Lehigh’s fluid labs. 4) If so, does that imply that I should not use 5w30 (my first choice) ?
5) Since my manufacturer recommendations do not take synthetics into consideration (or the existence of 5w oils for that matter), is it fair to assume that the recommendations are not appropriate?
6) What weight oil should I use… (1984 Toyota Supra and 1993 Plymouth Voyager)?
7) Do the new “SH” or “STARBURST” oils perform significantly better than the “SG” oils tested in the original post?
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