I would think that you will still get at least 8 MPG. It really does not cost that much to run and of course you would need to do that for the AC. As far as useful life you will never wear it out. They will run with very minor maintenance for many thousand hours. I like to keep the oil changed as well as the filter. It is better to over service it and it is also better to run it under load then to let it sit. Even if you don't plan to use it you need to run it for at least 45 minutes per month under load. Just go and have a great time.