Tech Tip - Motor Oils

By: Ed Hackett

While I hesitate to add to the traffic jam of Information Highway metaphors being proliferated by the media lately, I found the information below while speeding along the Infobahn. Given last month?s Tech Tip on "Fluids - The Life Blood of Your Ferrari," I thought this article on motor oils would make a good follow-on. As far as I know, the information is accurate but all the standard disclaimers apply. Jeff(Ed.).

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.

First, I will answer a couple of questions asked by another. Yes, it is OK to mix mineral and synthetic oils. One of the early synthetics used was a Polyalkylene Glycol. This was totally incompatible and would gel when mixed. This has not been used for years for automotive lubrication. All common synthetics used for engine lubrication now days are a Polyalphaolefin (Mobil 1) or a Dibasic Organic Ester type (AMSOIL). These are fully compatible with conventional oils. In fact Golden Spectro and AGIP Sint 2000 are mixtures of mineral and synthetic oils. It is always best to mix oils with the same rating (SG). This insures that the additive packages are compatible and will maintain their effectiveness.

All engine oils use an organic zinc compound as an extreme pressure/anti-wear additive. Spectro adds more to their Motorcycle oil than to the car oil because zinc is a poison to catalytic converters. You will also see that some "car" oil contains more than their motorcycle oil. The difference in zinc content between .11% and .16% is insignificant to the converter. The little data I saw on the oils packaged by the motorcycle manufacturers indicated that they were no better than the top automotive oils. (Many are in reality just repackaged and in some cases slightly reformulated top grade auto oils).

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 database.

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 the measure of how thick an oil is. 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.

Multi viscosity oils work like this: Polymers are added to a light base (5W,10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow<

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