One more...here's something from Hot Rod:
Motor oil gets a certain amount of contaminants in it as a vehicle is driven. The contaminants include moisture (water), dirt, wear metals (such as iron and aluminum), and unburned gasoline. Additives in high-quality oil are designed to hold contaminants in suspension, counteract acidic reactions, and provide a good support product to lubricate and cool vital engine components. But these additives can only do so much before they wear out or are used up. Obviously you should change oil before this happens, but how do you know when this point has been reached? Here are a few tips, courtesy of 76 Performance Products, the racing gas (and oil) guys:
* Short-trip driving is indeed severe service and requires frequent oil changes because the oil does not get hot enough to evaporate the water or gasoline contamination that gets in it. (Note that gasoline contamination is generally more severe on a carbureted car than it is on a fuel-injected vehicle.) Vehicles subjected to short trip driving (three miles or less) should also get an occasional 15-20 mile trip to drive off the contaminants.
* Special-interest (seasonal) vehicles like street rods and musclecars based in the northern states are usually stored during undesirable weather periods for three to six months. You should change the oil and filter before putting the car "away" for the Winter. This gets all the moisture, dirt, and unburned gas out of the crankcase.
* Starting the engine of a vehicle in storage every week or two sounds like a good idea, but is actually very hard on the oil because the oil rarely gets hot enough to drive off the new batch of contaminants. They continue to build up and at the end of the storage period, the oil may be in fairly poor condition. Consider removing the spark plugs, squirting oil in the cylinders, and periodically rotating the engine by hand so the valvesprings don't take a set (stop rotating the engine at a different point than where you started so the valves that were open are now closed and vice-versa). Spinning the engine over with the starter is acceptable only if there is assurance that raw fuel is not getting into the engine.
* For the people living in mild climate areas that drive their cars occasionally throughout the year, accumulate no more than 2,000-3,000 miles during that period, and typically drive at least 15-20 miles each trip, changing the oil and filter once each year should be adequate.
* Synthetic oils get dirty and contaminated just like conventional mineral-based oils, so synthetics must be changed at regular intervals, too. The primary performance (as well as any longevity) benefits of synthetic oils is their better low-temperature flow characteristics and greater stability and resistance to break-down at high temperatures. Synthetics must be changed as often as mineral oil when subject to the same contamination conditions.
* When changing oil, always have the engine thoroughly warmed up so the oil drains easily out of the engine when the drain plug is removed. This allows removing the greatest amount of oil from the engine in the least amount of time.
To reiterate, occasional startups are worse on the oil than leaving the engine unfired in storage. The best solution is to change the oil before putting the car into storage, then rotate the engine occasionally by hand. Assuming the storage period is limited to six months or less, the oil should still be OK at the end of the storage period and there's no need to change it. For really long-tem storage fresh oil should be adequate for several years providing the engine isn't started, but is stored over six months it's a good idea to change the oil before returning the vehicle to service. If the engine is started while in storage, fresh oil is definitely required at the end of the storage period, if not before the end of storage.
