i've been told by numerous technicians who have worked for dealerships that as long as you keep receipts for the oil filters you should be fine with resale. this is from a trade-in standpoint, maybe not for a private party sale.
i guess it makes sense if you keep receipts for filters because if you get a filter, you're more than likely changing the oil as well. can't really say that if you have records on buying the oil, cuz that doesn't really mean you have changed the filter. also, it's more black and white with filters because if you're buying those, you're probably changing the oil on the car it belongs to. if you're just proving you're buying oil, maybe you're doing it on another car. at least filters are more car specific than oil is
IMO, you're wasting time keeping any of that stuff. So you have a receipt to a Mazda3 filter. You're "probably" changing the filter on your Mazda3, but then again, when it benefits the dealer, you're "probably" not. Maybe you have a friend that works for Mazda and gave you a bunch of receipt paper from the parts department so you could counterfeit receipts. If you don't have service receipts from a licensed mechanic with your VIN number on it, it's worthless in a court of law, if that's what you're concerned with. Then again, they've gotta prove you didn't change your oil and the manual says nothing about having to get it changed by a licensed mechanic. It's just recommended, although it may say something different in the fine print of our warranty, which I have yet to see or read.
As far as having proof of maintenance for someone coming to look at your used car for sale, I'm big picture. If I go look at a used car and I get there and it's beat to hell, the interior looks like the floor at a Taco Bell, the engine bay looks like noone had popped the hood in the span of ownership and more importantly, the seller is a nutball or has a bad attitude, I'm not going to buy that car. I've been a car fanatic my whole life and if you were to come look at my car for sale, you'd know that I took care of it. Granted, anyone can spit shine a car up to look presentable for sale or trade in, but you can tell whether or not someone is the type to take care of a car and if that person handed me receipts to random parts they may have bought online, I would be thinking, "Ok, so?". Would it be icing on the cake? Maybe, but again, I can tell when I look at and drive a used vehicle if it was taken care of and pieces of paper that may or may not be legitimate are probably not going to sway me either way.
I for one take just about ANYTHING someone at a car dealer tells me with a grain of salt. Whether it's a salesman, service manager, technician, you name it, in my eyes, they're all crooks until I see it in writing. Sad, but there was a day, way back when I was young and stupid, where I actually did trust them and I've learned since then. All they're concerned with is moving cars and making money and I guarantee that your stack of oil filter receipts in the heat of the battle aren't going to mean jack squat when the dealer claims you haven't maintained your vehicle in accordance with the owner's manual. That's business at a car dealer. Doesn't mean you can't stand your ground, get a lawyer, fight it and win, because you most likely could and they'd most likely back down, but if they want to give you a hard time, they will. If a technician told you that and you get your car serviced there, ask him to put it in writing.