Engine oil gets contaminated over time by moisture, fuel, and particulate matter.
Moisture generally being introduced as a byproduct of combustion, it is easily burned off by getting the oil up to temp. If you have a short commute, it is a good idea to take a long drive at least once a month in an effort to get the oil to a more reasonable temperature.
Fuel contamination is generally a result of cold starts. We generally gauge the life of the oil and the change interval based on mileage, but most of the degradation and contamination of the oil with short commute vehicles is a function of cold starts. It is best to crank the car and start driving immediately. This will cut down on the warm-up time and reduce the total contamination from the cold start event.
Particulate matter in the oil gets there one of two ways. The first is via the air filter. The air needs to be as clean as possible going into the engine. The fine particulate can cause engine wear. The oil filter will catch a lot of it eventually, but that may be after it has spent some time on the side of the cylinders causing wear. We recommend either paper filters or multi-layer foam such as is found in our intake kit. The other particulate matter in the oil is generally a function of the wear. This will be ring material, cylinder material, and possibly bearing material. Some wear is normal over time, especially on cold starts when the parts of the engine are not their ideal shape or size. Catching some of that wearing material before it gets run through the oil pump can help reduce some of the secondary effects, so a magnetic oil drain plug is a good idea.
An appropriate oil change interval with a suitable oil will be the best way to prolong the life of your engine. Not every application is the same, so it is a good idea to consult with experts regarding your particular needs. Collecting an oil sample and sending it off for analysis is a great way to see if you need more (or less) frequent oil changes as well as keeping an eye on what is going on with your motor over time.