The paper presents a method for determining the optimal interval for preventive maintenance/replacement using either an age related or a diagnostic related renewal strategy. Additionally, the authors rise the question: "How does preventive maintenance influence the probability of failure and the mean life-time of preventively maintained elements of a technical system?" They answer the question using analytical and simulation computing approaches to arrive at the solution. The results are in quantitative form. giving relationships between preventive maintenance intervals and survival probability functions and mean life-time characteristics. Examples demonstrate suitability of the method for typical engineering components with Weibull life distributions. Applications offer substantial benefits to both the manufacturer and the user of technical equipment.