TractorPal Maintenance Logging app
This app has some potential and might be worth a look. The slim version is free; the upgrade is $10. There are limits to what you can add and how you can track, so maybe it isn't for you, but it might be worth a look.
Machinery logging apps are probably the wave of the future, and will make it easier to get the right part number, oil filter, and so forth.
It looks like you can put in three items before they want to start charging you. I might try a couple and see what it looks like before I pay any money.