The longer motor oil sits in a crankcase, the more it gums up or otherwise deteriorates. Even if you drive less than 100 miles a year, you need to change the oil as well as check the filter element (if you have a replaceable element filter as many K-F products use) for degree of dirt or filth. Also the brand and type of oil is important as many of the current brands and standard types of oil (SL, SM, etc) are formulated for the metals and use conditions of late model engines not the older cars. The lubrication people at Ford Motor Co. recommended either Quaker State or the Shell Rotella oil (originally intended for trucks and farm equipment) for older car engines (1940's, 50's etc) regardless of make or type. They still contain trace elements of minerals that the metallurgy of older motors need for proper long-term life.