Oil changes take 20mins, I'm not sure how that equates to a whole Saturday. Hell, you could do it on your way home from work. Or if you find a nicer shop (ie. a local, nonchain shop), they'll even let you drop it off in the morning, drive you to work and then pick you up from work to get your car in the evening.

Unless you make many hundreds of dollars an hour, the opportunity cost of 60mins (there + back plus change time) is rather low