Like some of you have said, I keep everything including gas receipts with total mileage, trip mileage, and mpg. All service records (done by year, not mileage). I change the oil yearly between scheduled services and save the crush washer and gasket.
I keep meaning to put this info into a spreadsheet, maybe tomorrow.
I keep the owner's manual in the glovebox because I'm older than many/most of you, and I'm never sure I've memorized anything anymore.