It is best to rely on oil analysis to determine your oil change interval. Of course, on many smaller vehicles that only take 4-6 quarts of oil, the cost of the oil analysis can be nearly as costly as a complete oil change.

So, for many passenger car owners, the best thing is to utilize a “do it yourself” oil analysis. It is not likely to provide results as accurate and precise as the ones you’d get from a lab, but it can give you a fairly good idea of just how effectively your engine oil is holding up.

In the next few paragraphs you’ll find detailed instructions for one of six DIY oil analysis tests that you can use to establish the condition of your oil and whether it’s ready for a change.

It would likely also be in your best interests to learn a bit more about lubricants and filters in general. Quite alot of information can be gleaned from the following sites.

- The Motor Oil Bible – tons of motor oil information

- The Motor Oil Evaluator – motor oil comparison specifications

- A Motor Oil Forum for discussion of motor oils

The Business Card Test

This test reveals oxidation products, sludge formation, dispersancy failure, glycol contamination, water contamination, fuel dilution, and high levels of particles.

While your engine (and the oil) is WARM (not HOT), pull your dipstick and place one drop of oil on a heavy, white, NON-glossy business card. Lay your “oil drop test strip” (the business card) flat, but so that all but the very ends of the card is suspended. For instance, 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 coffee cup.

Wait until the oil drop is no longer moist. Use the list below to evaluate the condition of your oil based on the DRY oil spot.

- A colorless circle or somewhat yellowish outer ring = “good” oil.

- A dense, dark deposit zone = Dispersancy failure

- A black, pasty area = Anti-freeze in your engine oil

- Center of circle dark with distinct outer ring = Severe oxidation

- A dark center with surrounding rings = Fuel in oil, Fuel dilution

Information for this business card test in: Fitch, J.C., “The Lubrication Field Test and Inspection Guide”, Noria Corporation 2000