This was cut and pasted from my local tuner forum. I thought it was interesting and may be useful to this forum's membership. Feel free to share your thoughts on what's written below...
"It is often difficult to know for certain just how long your oil could last before needing a change. The type of car you drive, the size of the engine, the age of the engine, the type of driving that you do and the type of oil that is in your crankcase will ALL have a significant effect on oil longevity.
Enter Oil Analysis
The best way to establish realistic oil change intervals is via oil analysis. Those who have been professionally trained to test oil and who have expensive lab equipment at their disposal are certain most qualified to determine the quality of the used oil sitting in your vehicle.
Such a thorough analysis of your oil can be cost prohibitive, though, since a professional oil analysis can often cost as much as a 5 quart petroleum oil change.
Below you'll find detailed instructions for 1 of 6 layman's oil analysis tests that you can use to determine how well your oil is holding up and whether it's ready for a change. In this way you can begin to set realistic oil change intervals for your vehicle.
Performing the Test
Using just this simple layman's oil analysis test can shed light on a wide range of potential oil problems which could require an oil change: excessive particulates, condensation build-up, glycol contamination, fuel dilution, failure of dispersant additives, formation of sludge and oxidation products. It is probably one of the most useful DIY oil analysis tests you can perform, and it's drop dead simple.
While your engine (and the oil) is WARM (not HOT), allow a drop of oil to fall from your dipstick onto a heavy, white, NON-glossy business card. Lay the paper or business card flat, but so that all but the very edges of the paper is suspended. As a possible example, if you're using stiff card stock or a stiff business card (which you really should be) simply set the card across the top of a cup or mug of some sort.
You want to wait for the paper or card to absorb the oil drop completely which might take awhile. The list of characteristics below should help you evaluate the condition of your oil based on the DRY oil spot.
-If your oil is still good for continued use, the dry oil spot will be uniform in color without any especially dark areas or rings. There may be a slightly yellow outer ring.
-If your dispersant additives are failing, you'll likely see a very dense and quite dark area, normally within the center of the circle. Consider changing your oil soon, especially if any other issues come up in the course of "testing".
-Glycol (antifreeze) in your oil? Expect to see a very black and somewhat "pasty" zone within the oil spot. Change your oil very soon.
-If the circle is really dark throughout and has a very distinct outer ring, your oil is severely oxidized and needs to be changed immediately.
-If the center of the circle is quite dark and there are outer rings you may likely have fuel in your oil. This does not necessarily mean that you need to change your oil since it is common to have fuel in your oil, but it could if the level is too high. Only a professional analysis will tell you how high those levels are."
Here is something I found when I researched "Marvel Mystery Oil" after reading that a Vortex member was getting a few MPG better after using it. According to this research text from the University of Waterloo, gas and oil additives are a waste of money. I encourage you to read this interesting study paper here: