Are we talking about a new or existing customer? Either way, I think the real answer is a loyalty program. I know there's programs out there that companies can subscribe to and I'm sure ones that are targeted towards dealerships (I'm to lazy to google it), but should they get free oil changes, yes .. discounts, yes. Like anything, it all needs to be tracked though for and IMO built in a nice interface that the service writes/advisers can use.
With all that gibberish said, I think oil changes are the quick fix - Assuming it's at least semi-convenient. Most people will return there just for the oil change. They barely cost anything for the dealer. Oh, and it obviously gives them a reason to bump the upsell the customer.