Once they are out of warranty, service hours really don't matter. You wait until it fails, have starting or running issues. Bearings usually warn you, and that is the main reason of the service. If you aren't using it to fly in a model, I wouldn't think that it matters, especially if you are looking for an older engine that is already out of warranty. Buying used, make sure you see it in person and see it run. The life time warranty engines you must meet their service requirements to remain under warranty. Just know that those engines aren't truly warrantied for life. One, when they build new model of engines, those companies don't make parts for the older and the obsolete ones. And, warranties are only as good as the longevity of the company. Which in this business is rarely a life time.