I have learned people will take anything you're giving away for free just as long as you are willing to do it. And once you do it, they expect it.

An oil change should be considered a service. Maybe include a "free" tire rotation if you want. Just make sure the time is covered. A quick glance at the brakes and tires while doing that. Look around for leaks, stuff falling off, belts, lights, while the oil is draining and while you're adding oil and topping fluids. It really doesn't need to be an hours long production dissecting everything. People soon figure out whether you're looking out for them or for yourself.

An in depth inspection is offered at some places for a set price, maybe an hour. Often it is sold as a used car inspection service or a Spring get the car ready service.

Doing annual inspections are when you really look for needed stuff you can sell, especially since it HAS to be fixed.

Two of the best pieces of advice I've received is "If you're good at what you do, don't do it for free." And "Sometimes you're better off firing a customer." The 19.95 oil change - free inspection places are a magnet for tight wads, cheapskates & FSA types who want someone to give them everything. They go to the auto parts for the "free diagnostic" that ain't worth spit, buy the part and expect you to install it for $10. Then want you to fix it for free when it doesn't fix the car. I know all about that from both the shop and the auto parts side of it..

This guy started out fine. OP was comfortable with the guy. Then the guy went into "get as much as I can out of this guy while the getiin's good" mode, tried to BS him and ended up screwing himself. Do good work, get paid for it, don't BS the customer or take advantage of them. If they are a regular and not a PIA an occasional favor for goodwill is fine. And if "Mr. I did you a favor and brought my own parts" shows up, send him down the road.

If the results don't match the theory, change the theory.

EXACTLY!