My friends often make fun of my OCD. I look after 5 vehicles in the family, each gets their own spreadsheet stored on the cloud (OneDrive). All service items (filters, oils, etc...) are sorted by column to track when things have been done, when they need to be done next, what parts were used, inspection notes, etc..
I also keep comments on things like fluid spec, quantity, and misc learnings/tips. This also includes all torque specs from service manuals.
Lastly, I keep documented the various parts I might want to keep an eye out for next time I visit the junkyard, or new parts for the upcoming services on the 5 vehicles that I can bulk buy from RockAuto.