When was the last time you had your oil changed? Oil is one of the most vital components in any engine. Routine oil changes are the key to your engine lasting a very long time. There is an old saying, "regular oil changes are cheap insurance". So the question is how often should it be done? The answer depends on what type of equipment you have and how you use it. But here are some rough guidelines. On most small engines, like lawn mowers, I suggest a minimum of once a year, even if you don't use it very often. However, if you are using your equipment for extended periods of time (for example if you have a large lawn or you are mowing a field of some sort), in hot and dusty conditions or on rough terrain it is important to do it more often. If your engine has an oil filter be sure to change it as well and follow manufacturer recommendations for oil viscosity.
Motorcycles and ATV's should be done more often depending on how you use them and how much. The more oil your engine holds the longer it can go between oil changes. Many Japanese dirt bikes hold somewhere around 1000 cc's of oil. If you are doing lots of hills in hot and dusty conditions you should change your oil more frequently. On my personal bike during the summer I change my oil every 100 to 150 miles of off-road use. I use a washable oil filter to keep the cost down. Which means an oil change is about $5.00 and takes 10-15 minutes, again, cheap insurance. During the winter months I change it less often since the heat and dust are less of an issue.
Also, it is a very good idea to check your oil level weekly and note the color of the oil on the dipstick. If you notice that the oil appears watery you may have a problem with fuel dumping into your crankcase which can lead to premature engine failure. If your oil is black, it's definitely time for a change. Clean oil is generally amber in color and gradually darkens as it collects carbon and bits of metal from the engine. Typically oil will start out amber and slowly darken in color.
Having your oil changed or doing it yourself can be a hassle and easy to put off, but having to shell out money for new equipment (not to mention going through the buying process) is much worse.