Over drive is just that, over drive. It's an additional final gear and will not damage the car one way or another but will cause you to use more gas b/c it will not have the extra gear to go into when it can to save gas. And obviously since you have the potential of keeping the engine revving higher b/c of the lack of an over drive gear, there is a very slight possibility of a little more wear. But it's nothing compared to not doing your regular scheduled maintenance.
You either need a valve job or perhaps a tuneup and regular maintenance will help it if you haven't done that for a while.
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