As internal combustion engines for passenger vehicles have been forced to become more fuel efficient, less polluting, smaller, and longer lasting, their lubrication needs have changed dramatically. These changes in the engines have required the development of multi-weight detergent motor oils that are suitable for the tighter clearances, higher heat, and higher RPM of these engines. These oils have additives to reduce wear (especially at start-up), maintain viscosity, and to suspend the soot and contaminants (by-products of combustion) that they wash off the interior engine parts. A side benefit of these oil's ability to maintain viscosity and suspend soot is that the oil change intervals have become longer on most vehicles.

Sludge

The number one reason for oil changes is to prevent the formation of sludge. It's important to understand how sludge forms and how sludge formation is prevented.

How Sludge is Prevented
Additive in the detergent oils 'wash' any contaminant particles off of internal engine parts and hold these particles in suspension until the oil is changed. It's important to understand that these particles are too small to cause engine wear, but they do turn the oil a darker color. The key thing is to change the oil before the oil becomes too saturated with contaminants to hold any more, but color is not an indication of this condition.

How Sludge Forms
As explained above, modern detergent oils suspend contaminants so they do not settle on engine parts and form sludge. When the oil becomes saturated with contaminant particles new particles settle out of the oil onto the internal engine parts and form sludge. This is why it is so critical to perform oil changes before this level of contamination is reached. The only way to know if you're oil needs changing is to have an oil analysis done. Absent this, play it safe and follow the severe service interval specified in the owners manual. Remember that the contaminants come from the combustion process of the gasoline and and the air. Synthetic oil will not prevent these contaminants. Synthetics may be marginally better at suspending more contaminant particles, but not enough to prolong oil changes by much.

Oil Analysis
To determine the optimum oil change frequency for your vehicle requires that you perform several oil analysis during one oil change interval. For example, if your vehicle has 15,000 miles on it and the manufacturer recommends 7500 mile oil changes for normal service and 5000 miles for severe, perform an analysis at 18,750 miles, 20,000 miles, and 22,500 miles (if the first or second test shows a need for an oil change then stop there). Do not exceed the manufacturer's normal service interval even if the analysis shows no need for an oil change at 7500 miles. The oil change industry desperately desires that you NOT perform such an analysis. The almost certain result for most drivers will be that even at 7500 miles the oil will still be fine.

Even after your vehicle is out of warranty it is a good idea to continue to follow the manufacturer's schedule for maintenance. There are frequently special campaigns (not recalls) to fix latent defects after the warranty has expired. Lately we've seen these on some Toyota V6 engines and some Saturn engines. You want the manufacturer to have no excuse to deny coverage. Also you can sometimes get a manufacturer to share the cost of an expensive repair when something fails after the warranty has expired, but this is at their pleasure and it is best to have solid proof that you have followed the maintenance schedules.

Big oil users like bus companies and truck fleets use oil analysis to extend the life of their engines without unnecessary oil changes. The reasons are clear. These big engines can use 3-4 gallons of oil and unnecessary changes are expensive in both time and materials. In some cases they change the filters and put in additives to replace the acid neutralizers and anti-wear agents. A good analogy is swimming pool maintenance. You clean the filters, you remove the debris, you add stabilizers and disinfectants, but you rarely empty the whole pool and refill it.
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