Brake fuid question

[JSharpe]

I just finished a C5 brake upgrade on my 71 camaro. I put prestone synthetic DOT 3 brake fluid in it. My master cylinder started leaking, so I am swaping it out. Can I put non-synthetic DOT 3 back in and let it mix with the synthetic? Or should I keep using the synthetic DOT 3? Or, could I flush the system with DOT 4 and just use that? Sorry for all the questions, I appreciate the information. Thanks, Jonathan