Why would you do that?
Just as easy to "check for leaks" with the synthetic, and the system would not be contaminated w/ 3.
Using 3, and having leaks
can/will ruin any paint it comes in contact with.
If you are thinking "cheaper".. Not, as you will have to pump a considerable amt of syn thru to remove the 3....
But, that's just my $.02.. reality may differ.