Just to add here that if you top up using the TfL Oyster App the minimum you can top up with is £10.
You can only do a minimum top up of £5 on the website for some reason.
I actually find the lack of ability to be able to top up online with any value above £5 to be a nuisance as I feel it would make more sense to be able to top up £5.25 to cover a day cap on the buses and trams.
Chris