The first oil I remember buying is M1 5W30 as a topoff for my RAV4.
My first car was a '76 Nova that my dad gave me, think that there was a jug or two of oil he left in that massive trunk but I never checked the oil myself. Until I started changing the oil in my FXT last year, I expected that the dealer or quickie change place would tell me if I was down a lot when I went every 3000 or 3750 miles or whatever. I first bought oil because I had a leak in the RAV4 and wanted to be prepared, but I thought it looked more like ATF and it did turn out to be in the transmission ($3500 repair).
Looking back, I should have been checking my oil more closely, particularly in that Nova. I just did not even think about it.