FuelLog -- does what you ask, partial fill ups, space for notes on a fill up, which is handy, as I put in what kind of driving it was -- city, hiway, towing, etc. You can log more than one vehicle on the app.
I've been using it several years. I believe there is a free and a pay version. I tried the free version, liked it, so bought the pay version -- wasn't very much, maybe 1.99. The only thing it doesn't have I wish it had, is I wish it coordinated with a website, and transferred the data there, as when I change phones, I lose the data unless I want to re-enter it all on the new phone, which is tedious to do.
