Changing lubricants and fluids eats up a lot of the time and money you put into your regular preventive maintenance program. So it figures that if you can safely extend the intervals at which you change oils, greases and fluids, you'll save money and time. The key word is safely; we'll explain how below.

Trucks operate in a dirty environment. Splash and spray toss road film and chemicals into the air, microscopically fine dust enters the combustion chamber, and diesel combustion itself creates soot and acids.

Oils and greases protect moving parts from the effects of contamination, but are consumed as they do so and eventually have to be replenished or replaced.

Take engine oil. It lubricates, cools, cleans and protects the insides of our engines. It must function in an environment where clearances are far less than the diameter of a human hair. In this environment, contact between cam shafts and lifters or meshing gear teeth generates forces measured in tens of thousands of pounds per square inch.

Oil survives to do its job, thanks to chemicals added to keep it from breaking down. The chemicals also help increase fuel economy by lowering friction where parts rub together.

At the same time, combustion creates soot. It can abrade engine surfaces, or react with oil additives to neutralize them. Additives also neutralize acids, and are consumed as they do so. Filters remove only some of the contamination. Some is held suspended in the oil, circulating as particles too small to do significant damage.

Eventually, though, filters clog and oil additives are consumed or reach their maximum capacity to protect. So, to prevent damage and breakdowns, we have to change oil and filters as a major part of preventive maintenance.

PMs take time, not only to perform the service, but also to get to and from the shop and to wait for it to be done. This could take an hour or two at a truckstop, or half a day or more at a dealer or independent shop. If you can extend the interval between PMs, you can reduce scheduled downtime by hours or days per year. That means more uptime, and more revenue miles.

Manufacturers set conservative recommended maintenance intervals in order to protect warranty. You can safely go beyond these recommendations. By combining the following methods, drivers may go 50,000 to 60,000 miles between PMs. One fleet tested to 100,000 miles, but decided it didn't want its trucks out that long without having other things inspected.

To extend oil drain beyond manufacturer's recommendations, you should:

Use oil analysis. This service tells you what contaminants are in your oil and how much life is left. Wear metals should be compared with allowed levels published by the engine builders for each model and rating. TBN (total base number) measures acid-neutralizing alkalinity. It also indicates how much detergent and dispersant additives remain. Oil analysis will also tell you if there is fuel, coolant or dirt in your oil.

Plot your results. Take a sample at what would normally be your drain interval, but don't change oil. When the report comes back, decide then whether to continue without an oil change. Take further samples at half-intervals. (If 20,000 miles is recommended, do the first at 20,000, then every 10,000 after that.) Track results. If they approach critical levels, change oil as soon as practical. Many labs will post your results on their Web sites. With a password, you can view your results immediately. Some will even graph your progress.

Try to use the same lab. Stock up on sample bottles and shipping labels. Lab procedures vary slightly, so for consistency, use only one.

Look into oil companies' programs. These include Shell Care oil analysis, Chevron Lube Watch, Pennzoil oil analysis, Mobil Delvac AccuTrack oil analysis program and Exxon SmartLube. With locations nationwide, they're convenient to use; stop in and ask about their programs.

Use premium or synthetic oils to extend intervals. Premiums have more additives in them, and synthetics are more stable.

For extra-long intervals, use off-line by-pass oil filters to supplement spin-on full-flow filters. There are several types of by-pass filters: centrifugal, compressed fiber media and wound or wrapped fiber or paper media. Check manufacturers' claims. Most filter to at least 5 microns, compared with 25 microns for the best spin-ons. Some clean to less than 2 microns.

Keep copies of your oil analysis reports. One owner-operator had a warranty claim denied because he went 58,000 miles without an oil change (using synthetic oil and a wrapped-media by-pass filter) when a con-rod bushing broke. He brought out a stack of analysis reports and asked, "Which of these shows that I should have changed oil?" His warranty was honored.

A final reminder: PMs are also a time when technicians or owner-operators check belts, hoses and other components more thoroughly than during pre-trip inspections, and when chassis parts get greased. You should continue to do this at the recommended intervals, even if you are extending drain intervals.