My rule of thumb because I never put more an 5k a year i change my oil every 4-6 months and have every paperwork for each of my cars. When I sell the car I give it to them and if they don't want it I throw it out at that point. I have about 4500 miles on my cobra now and it's going tue for it's 9 month service and it's 2nd oil change. My Tribeca had 10k on it after owning it for 2 years and it had oil changed like 6 times and never put anything less than 91 in it. As for looking at used cars you can usually tell when you look at it and how it runs. If they refuse to show paperwork less likely the were that **** about it.

 

I do keep a record of when I do any maintenance, but I never keep my receipts for such things like oil.