i am not sure what you mean by this, but the implication does not seam right to me? you are saying you have to be old to know when to change your oil? or only the old people have the time to remmber when the car's oil should be changed..? either way I don't get it..

The car's oil is changed variabley depending on the way you drive, what filter you use, and the conditions you drive in.. it is impossible to tell when exactly is the ideal time to change oil.. For example I don't follow my FSS and change oil roughly every 3-3.5k miles and after each change I write down the date and the mileage of when I did the change.. Some follow FSS and change every ~7k, and others go as much as 9-10k..

For example if you driving in a more congested with much stop and go you should change more frequently than if you drive in highways at stable speeds, if you use a paper filter you should change in shorter interval than if you were to use a fleece filter, if you are often riding in high RPMs you should change more frequently..

If your engine was replaced you should be follow break in procedure and I recommend an Oil change within the first thousand miles...at least