You have to be vigilant. Years ago, I received a promotion for 30,000-mile scheduled maintenance on my 92 XLT from my Ford dealer. It had so much stuff that was left out (or inappropriately added), I decided to investigate this for the newspaper I was working for. I checked similar promotions for my colleagues' cars of different makes and models. I found the same thing again and again. In some cases it was because the hire marketing firms to hawk generic service plans.
You can see the story here, if you like:
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("Car maintenance fraud").
Here's a newer version I did last year for another publication.
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Note a dealer's effort to change the timing belt on a customer's Pathfinder at :60,000 miles instead of the recommended 105,000 miles. Just as you said, after the customer refused, the dealer gave her a receipt with a warning that failure to follow recommended service procedures can invalidate the warranty!!!!
For my 92, I decided to get all the Rotunda tools and the Haynes and Ford shop manuals and do every bit of maintenance myself, including repacking the wheel bearings, greasing the driveshaft slip yokes (all three!) and all the other stuff. And I'm no auto repair expert. I saved money, and I know it was done right. And beyond that, it gave me a great feeling of accomplishment!!!!!! I've done it twice now, for 30,000 and 60,000 and all the lesser service in between.