I was also taught が only. The grammar dictionary, however, says “In general, the choice between が and を seems to depend on the degree of volition expressed in the action the experiencer takes. That is, if his volition is high, を is preferable.” I love how they use “seems”, like they’re only guessing from what looks to be common usage. They use the following sentence as an example of a weird place to use が:

私はやめようと思えばいつでも今の仕事をやめられる。 (I can quit my current job whenever I want to quit.)

They also add that できる always uses が, and that the spatial を (i.e. 公園を歩く walk through the park) and the detachment を (i.e. うちを出る leave home) are never replaced by が.