Can you use regular brake oil after using synthetic?

DOT 5 is compatible with DOT 4.

It is better to not mix them though.

You don't oil brakes. Brakes use fluid in their hydraulic system. DOT4 fluid is silicone based, DOT3 is mineral oil based. The two are not interchangeable. You use either one or the other.

you can but using synthetic oil is better for your engine.

it flows better across a wider range of temperatures meaning your engine is better protected when just started, when its very cold out and when it is very hot (driving on a track if you are into that).

The oil also lasts longer and doesn't start to break down as quickly and as much when nearing end of life meaning you get better protection though the entire life of the oil.

EDIT: im an idiot. i totally misread the question